Your exchange was interesting to me and I looked for rules for deaf competitors with no success. I did see a Chicago Deaf Spelling Bee in which the contestants were given the word in sign language and then had to spell the word with finger spelling... As for some of the words used in the bees I don't know if they would really have a sign language equivalent except through finger spelling.
The only thing I can think of would be to give the deaf students the phonetic spelling of a word on paper and ask them for the correct spelling? You raise an interesting question coloradan. (o: