The following worked for me with Ansible 2.1.2.0:

- name: Define list of mappings
  set_facts:
    something:
      - name: bla
        mode: 1

- name: Append list with additional mapping
  set_facts:
    something: "{{ something + [{'name': 'blabla', 'mode': 1}] }}"

What about this ?

- name: get all settings
  set_fact:
    all_settings="{{ my_list|map(attribute='settings')|list|flatten|unique }}"

So the way I figured it out was to put the when condition into Jinja.

- hosts: localhost
  vars:
    default_thing_1: 123
    default_thing_2: 456
    default_transport: train
    list_of_things:
      - name: foo
        common_thing_1: "sam"
      - name: bar
        common_thing_1: "sam"
        common_thing_2: "ham"
      - name: biz
        common_thing_1: "eggs"
  tasks:
    - name: Set the Things Facts
      set_fact:
        thing:
          i_am: "{{ item.common_thing_1 | default(default_thing_1) }}"
          eggs_ham: "{{ item.common_thing_2 | default(default_thing_2) }}"
      loop: "{{ list_of_things }}"
      register: things_fact_results
    - name: Set Optional Things Facts
      set_fact:
        thing: |
          {% if item.common_thing_2 is defined and item.common_thing_2 == 'ham' %}
          { "transport": "{{ item.transport | default(default_transport) }}" }
          {% else %}
          {}
          {% endif %}
      loop: "{{ list_of_things }}"
      register: transport_fact_results
    - name: Debug the Facts
      debug:
        msg: "{{ item.0 | combine(item.1) }}"
      loop: "{{
        things_fact_results.results |
        map(attribute='ansible_facts.thing') | list |
        zip(
          transport_fact_results.results |
          map(attribute='ansible_facts.thing') | list
        ) | list
      }}"

This is kinda sloppy but does allow me to specify the else condition to set the result over every iteration to something instead of AnsibleUndefined.

I don't really like this solution but it works.

$ ansible-playbook playbook.yml 
[WARNING]: No inventory was parsed, only implicit localhost is available
[WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'

PLAY [localhost] *******************************************************************************************************************************************************************

TASK [Gathering Facts] *************************************************************************************************************************************************************
ok: [localhost]

TASK [Set the Things Facts] ********************************************************************************************************************************************************
ok: [localhost] => (item={'name': 'foo', 'common_thing_1': 'sam'})
ok: [localhost] => (item={'name': 'bar', 'common_thing_1': 'sam', 'common_thing_2': 'ham'})
ok: [localhost] => (item={'name': 'biz', 'common_thing_1': 'eggs'})

TASK [Set Optional Things Facts] ***************************************************************************************************************************************************
ok: [localhost] => (item={'name': 'foo', 'common_thing_1': 'sam'})
ok: [localhost] => (item={'name': 'bar', 'common_thing_1': 'sam', 'common_thing_2': 'ham'})
ok: [localhost] => (item={'name': 'biz', 'common_thing_1': 'eggs'})

TASK [Debug the Facts] *************************************************************************************************************************************************************
ok: [localhost] => (item=[{'i_am': 'sam', 'eggs_ham': '456'}, {}]) => {
    "msg": {
        "eggs_ham": "456",
        "i_am": "sam"
    }
}
ok: [localhost] => (item=[{'i_am': 'sam', 'eggs_ham': 'ham'}, {'transport': 'train'}]) => {
    "msg": {
        "eggs_ham": "ham",
        "i_am": "sam",
        "transport": "train"
    }
}
ok: [localhost] => (item=[{'i_am': 'eggs', 'eggs_ham': '456'}, {}]) => {
    "msg": {
        "eggs_ham": "456",
        "i_am": "eggs"
    }
}

PLAY RECAP *************************************************************************************************************************************************************************
localhost                  : ok=4    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   

Now the transport key is defined in the one object but not the other. So I can pass the item off to a complex object that's part of an API call.

I think I've found a cleaner, easier way to deal with these kind of things. Ansible runs all strings through jinja and then tries to load the result as yaml. This is because jinja only outputs strings so that allows it to load a data structure from a variable if there is one.

So any valid yaml in a string is loaded as a data structure -- so if you template valid yaml it will get loaded as data.

Trying to template correct yaml in the conventional, human form is tricky. But yaml loads all json. So, json is easier because there is no need to worry about whitespace. One bonus though, yaml does not care about extra commas, so that makes templating it easier.

In this case here is the playbook from the top answer rewritten to use this method.

- hosts: localhost
  vars:
    users:
      - { name: "user1" }
      - { name: "user2" }
  tasks:
    - name: Get uids for users
      command: id -u {{ item.name }}
      register: uid_results
      loop: "{{ users }}"

    - name: Show users with uids
      debug: var=users_with_uids
      vars:
        users_with_uids: |
          [
            {% for user_dict, uid in users | zip(uids) %}
              {
                "name": {{ user_dict['name'] | to_json }},
                "uid": {{ uid | to_json }},
              },
            {% endfor %}
          ]
        uids: "{{ uid_results.results | map(attribute='stdout') }}"

Notes

The | character tells yaml to load a multi-line string. Instead of putting the variables in quotes I use the to_json filter which will quote it and, more importantly, automatically escape anything in the variable that needs escaping. Also, remember commas after list or dictionary elements.

The results should be the same:

TASK [Show users with uids] ************************************************************
ok: [localhost] => {
    "users_with_uids": [
        {
            "name": "user1",
            "uid": "1000"
        },
        {
            "name": "user2",
            "uid": "1001"
        }
    ]
}

One more thing

I like to use the yaml callback especially for testing this. That way if my json-looking yaml doesn't get loaded I'll see a json-like structure. Otherwise it will come back in normal looking yaml if it was loaded. You can enable this by environment variable -- export ANSIBLE_STDOUT_CALLBACK=community.general.yaml.