I only just came across your project and this is something I've been wanting for a while. There are a few projects for the opposite sort of adapter but few that turn a USB keyboard into an ADB one.
Firstly, can your adapter handle 'combo' keyboard & mouse/trackpad style models? That's the type I would likely want to use (something small and combined to easily test my Macs with).
Secondly, I might be a bit late in replying if you've already designed the PCB, but would it be possible to have a USB keyboard with a media 'Power' key trigger the ADB power button instead of having to physically press that button on the PCB? Quite a few keyboards have that key (as well as the USB Mac models).
Finally, I would love to have a way to at least use some media key functionality under early MacOS. The Apple Adjustable Keyboard had 'Volume Up', 'Volume Down', 'Mute' and a 'Mic' button which doesn't really have a modern equivalent and could possibly be mapped to Play/Home/Browser/whatever.
TMK has
documented these multimedia keycodes now. You do have to broadcast a second device address (0x07). If you could have a mode that advertises the keyboard as an Adjustable then it would at least allow you to have keyboard volume control under OS7. (EDIT: Found a
report that says the media keys work on 6.0.7 or later)