Neil:
The important question is: what do our potential deaf users _themselves_ want? Then we can worry about who's going to find the motivation to adapt the software appropriately to support their needs.
Not to discourage, but I can well imagine that many deaf users are quite happy with what is already there. Sign language is probably their first language, but many will have their literacy in the spoken language - that's the language they read and write, often on a daily basis.
It's the same with many other speakers of dialects and smaller minority languages - the 'big' language is the language of schooling and written communication. Smaller languages often remain just oral languages - or in the case of sign language, signed languages.
Of course this is all spoken by someone who does not have any direct experience with deaf people, so I gladly admit I'm wrong if someone else with more direct experience tells me differently. :-)
Andre Engels