rsplit and join could be used to simulate the effects of an rreplace

>>> 'XXX'.join('mississippi'.rsplit('iss', 1))
'missXXXippi'

You could put the translations into a dict, and also combine the search-patterns into a single regular expression, which gives priority to the longer patterns. Then use the callback argument that re.sub accepts to make the replacement using the dict.

import re

trans = {
    "00": "a",
    "11": "b",
    "01": "c",
    "10": "d",
    "0000": "e",
    "1111": "f",
    "0101": "g",
    "1010": "h"
}

regex = "|".join(sorted(trans.keys(), key=len, reverse=True))

# demo
s =  "0000000110110110100111111111"
result = re.sub(regex, lambda x: trans[x.group(0)], s)
print(result)  # eacdbchcff

Just build a new string only substituting the 2-char substrings at even indeces:

repl = {
    '00': 'a',
    '01': 'b',
    '10': 'c',
    '11': 'd',
}


filedata = ''.join(repl[filedata[i:i+2]] for i in range(0, len(filedata), 2))

>>> def rreplace(s, old, new, occurrence):
...  li = s.rsplit(old, occurrence)
...  return new.join(li)
... 
>>> s
'1232425'
>>> rreplace(s, '2', ' ', 2)
'123 4 5'
>>> rreplace(s, '2', ' ', 3)
'1 3 4 5'
>>> rreplace(s, '2', ' ', 4)
'1 3 4 5'
>>> rreplace(s, '2', ' ', 0)
'1232425'

First of all, I think you're approach is ok. Oneliner, pretty pythonic.

The question suggested in the comments (Right-to-left string replace in Python?) provide several useful answers that worth measuring them.

One is the same approach you're taking: x[::-1].replace(' ', ' X ', 1)[::-1] and the other one is with rsplit and join.

I profiled both and this was the output I got:

$python -mtimeit -s "s = 'foo bar bar'" "s[::-1].replace(' ', ' X ', 1)[::-1]"
1000000 loops, best of 3: 1.18 usec per loop

$python -mtimeit -s "s = 'foo bar bar'" "' X '.join(s.rsplit(' ',1))"
1000000 loops, best of 3: 0.8 usec per loop

So my recommendation is the second approach which seems a lot faster. That being said, whichever you chose, put a comment beside telling # this is right replacing! because either way is pretty unclear what the code does :)

Hope this helps!

rsplit and join could be used to simulate the effects of an rreplace

>>> 'XXX'.join('mississippi'.rsplit('iss', 1)) 'missXXXippi'