Yes, I think a PC keyboard without Windows keys, or the legend changed to something else would classify as a Unix keyboard. Sun uses the diamond ◆ symbol on the same key that in USB is used for Windows/Command key and the Compose key is the same as the Menu key.
In the mid '90s, several Unix vendors started shipping keyboards with the "Enhanced" 101/102-key IBM PC layout with their machines. Keyboards were often from OEMs and some had PS/2 plugs. The Alps-made SGI Bigfoot
is well known. I have used both SGI-branded and DEC-branded NMB RT6856
with their respective Unix machines.
IBM RS/6000 running AIX had Model M's, I think, but I don't know if they used the "Terminal" layout and protocol though.
HP and Wyse has also made terminal keyboards with IBM's "Enhanced" layout but in their distinct styles and with a "Set Up" or "Select" key (for changing settings in the terminal).
I think a Tux key is just a gimmick though.
If you are going to adapt the Windows PC layout for Unix users, I think that you should change the Windows legend to ◆ and the Menu legend to Compose
to reflect how they are on Sun's keyboards.
If you are designing hardware as well, please do add a LED for Compose! There is support for it in USB HID. (Compose is also soo missing from third-party key sets, IMHO)