Has anybody considered using one of the Vinculum II based devices?
There's a few complete programmable boards as well as chips that can be used in combination with other hardware. I bought the Vinco about 7 months ago to make a Colemak-QWERTY converter dongle but got sidetracked with moving overseas and never completed it.
It's great to see a lot of other people motivated and interested to see the same product come to life.
Is there any reason something like the Vinco has been ruled out as an option? It's a bit steeper in price, but it has USB Host and Device capabilities, including HID support built into the on-board OS.