On a positional lane, why not put out four targets instead of two? Two of them will be the regular positionals, either kneelers or standers, and the other two will be at exactly the same place, distance and height, near as you can get, but will be freestyle targets, with 25mm kills for the "kneelers" and 15mm kills for the "standers".
Now, anybody and everybody can choose when they get to that lane, do you want to take a 30 yarder full size kill standing or a 30 yarder 15mm kill sitting? If it's a kneeling lane, either a 40 yard (for example) full size kneeler or a 40 yard 25mm sitting.
No need for proof of inability or lack of mobility, not much for a marshal to do, just elect which one to shoot at the time, and tell your buddy. Doesn't even need to be recorded which one you took, you either hit it or missed it.
Disabled and able bodied get the same choices.