Has anyone modified a front seat in the Cub or Champ to make it easier getting in and out for a bigger person?
If you can find a Taylorcraft L-2, take a look at the front seat. I know that they are scarce. (There may be pictures on the internet.) I friend let me fly his and I was surprised to find that the front seat is adjustable fore and aft. Later, I compared it to my Aeronca Champ and to the Cubs at the field. The T'craft installation is not that much different. It looks like a drop in replacement at first glance. At any rate, it looks copy-able.
Now for getting it blessed by the FAA? Hah! Seats are something that are looked at real hard ever since FAR23. They may require you to go through the same test standards as a new design. Best bet is to copy the L-2's simple design into a AB Cub clone.
American Champion has an adjustable front seat for the Citabria. You might get away with one of those, or at least maybe be able to get the FAA to bless it.
since it can greatly effect the CG of the aircraft, it will need a blessing from an IA. It will be up to him how to proceed. Talk with your local IA and see how he would attack the problem. Most likely a 337, probably a field approval depending on what you use for the seat. I don't see why the FAA would turn it down though, unless you homebuilt some sort of rail/seat.