No, not yet. It may be possible in the future, using 3-way calling, and a TDD modem, using a network that allows simultaneous voice and data. Alternatively, you can buy two phones (one for voice, one for data) and pull mobile VCO.
MiniTTY does work on the Treo600 and Treo650, though (confirmed.)
-- Helping add Real Time Text to Jabber/GoogleTalk/iChat networks!