You are welcome to write the letter, however, I don't expect it to get anywhere.
The STAC (the people who will review your letter) are leery of adding more RWD cars to STS and especially RWD cars with as good of a power to weight ratio as yours. The advisories committees as a whole also aren't generally in favor of giving favorable classing to old, rare, unsupported cars, because classing them favorably won't grow the sport.
So realistically I would suggest:
1. Stay in ES
2. Go to STR and just have fun, take every time you beat someone as a little victory
3. Get another car - your MR2 is nearly a collectors item these days, maybe it's time to retire it.