It's possible with time remaining during picks to open vault and SUPPOSEDLY select different taunt and announcer, but often (always?) it doesn't actually take effect until the next game after...
But I, for one, would actually like to be able to choose these options dependent on what hero I'm actually playing, as just another option like which avatar to use. I can't be the only one who wants this. The obvious incentive for S2 (besides how easy this would probably be to implement) is actually providing motivation for people to have more than one announcer and one taunt. Please consider!