@jesse and @algernon, thank you so much for your detailed replies. Good to hear that a port could possibly be done even without Teensyduino. And thank you also for the detailed info about the good practices of dealing with code from other FOSS projects.
QMK and Kaleidoscope share the same license which I would also use for newly written code of a possible port. That makes things at least a bit easier.
If I understand you correctly, the USB HID protocol is completely taken over by the A 32u4? Or are there any board specific parts of the Arduino HID library’s implementation that could cause trouble when being run on a Teensy board?
Do you expect any trouble with the upload? I know that some Teensy based keyboards use Teensy’s proprietary boot loader, others don’t. So I suspect that I will have to use Teensy Loader to upload to some keyboards (e.g. ErgoDox EZ). Or could that also be done with the Arduino IDE?