since several different kinds of accessibility can be provided, which kind of accessibility is needed also needs sorting out. Apple in 10.4 Tiger had things set up such that if the computer was completely hooked up including a set of speakers a question as to which language a user needed came up on the screen and this had a time limit on it. Failure to answer that question resulted in VoiceOver being turned on to interact with the user during installation and afterwards. Speech would be no help for those with no hearing ability. The deaf-blind usually have refreshable braille displays if involved in education or employment so perhaps detecting a refreshable braille display and activating it would help that group. Perhaps start by displaying a question in normal sized type. No answer, make the type larger until that's no longer possible. Next detect a braille display if present and display a question on that if available. Next, try speaking a question with espeakup and see if that gets a response from either a keyboard or perhaps a microphone if no keyboard is used. Perhaps a mouse response could be checked for as well somewhere in that group but not all people have or use mice. One thing that may be necessary to do since some systems these days are missing pc speakers or those are disabled would to play some tones on a sound card if one is found and perhaps adjust the post-install boot to play those tones on the sound card as well. We had to do this with some very recent hardware so my reason for bringing it up here.
One thing I really would like to see in a debian installer that installs a graphical environment is an option for the user if they choose to have debian start up in console then run ratpoison or startx to go into the graphical environment since if the graphical environment breaks they have at least logged in on the console and have a chance to repair the graphical environment without the need to destroy the whole installation and start from bare metal again. --
