I used plywood as the board in part of my board batten install. I was doing cedar with a clear coat. I used minwax spar urethane to coat the cedar and the plywood. The plywood went moldy in the first year mostly noticeable on the lighter areas of the ply.

If you are painting your board and batten I think you can use any wood. It is just going to be painful to cut the plywood into the strips. Normally someone doing siding calculates time and material into their equation and the time would dictate ordering something pre-cut from a lumber yard.

The other issue you'll run into is height. If you are going vertical then you probably need 10' strips so then you are looking at ordering 10'x8' sheets of plywood. I suspect if you cost this out then even before cutting your sheets of plywood are going to cost more than if you bought 1x4 @ 10' and 1x12 @ 10' of pine. Plywood is going to be more dimensionally stable but with paint grade material you can use whatever fasteners you want and make sure the wood can't move.