I would think the people persons involved in these projects:

http://kbdbabel.org/

http://xdb2usb.husak.com.pl/en/

Would be the best people to talk to, especially the first one. The second one is an adb-usb adapter. If adb decoding can be done, surely encoding can't be that much harder. Why not just disassemble an adb device and snoop on the communications or something?

http://www.retards.org/library/technology/computers/apple/macintosh/software/docs/inside_macintosh/ADB_Manager.pdf

Page 5-29 covers writing a device handler. This too maybe?

http://code.google.com/p/imate-osx/wiki/DevelopingANewADBDriver

It's for OS X, but maybe there might be useful info somewhere.