Well, funny you should mention software for USB and old Macs
As mentioned in
another thread, I'm considering building a NuBus carrier board for a FPGA, and one of the feature is a USB host connector (using a micro-B to save space) even though the primary goal is to create a framebuffer device. The USB works for a somewhat more modern bus in my
SBusFPGA design, where the FPGA includes an
open-source USB OHCI controller (so USB 1.1 only) that NetBSD/sparc can make use of (I only needed to write a SBus -> OHCI shim driver, the USB stuff is standard NetBSD; I didn't even think about good old SunOS or even Solaris). I only ever expected to try USB with NetBSD/mac68k and not System 7/MacOS 8.
I've yet to convince myself the board could work and order a couple. Seeed is likely over €500 euros for two populated boards, once custom duties are factored in... buying a lot more is a lot cheaper per-unit, but the design isn't proved yet so it's a risky bet :-( And I don't really need more than a couple anyway).
Another risk is that OHCI, even with just USB 1.1, is a bandwidth-intensive bus master. It doesn't quite overwhelm a 25 MHz SBus with reads handled by buffered 16-bytes burst, but I'm not sure that non-burst access will be fast enough over the 10 MHz NuBus...
For 68k Macs, I suspect the entire USB stack would have to be written, which would be quite a job (even if it were possible to reuse some *BSD code and stick to just HID and Storage at first). However the NuBus-based PowerMacs might have some support available in MacOS 9.1; *if* there's some OHCI support in there, then it *might* be possible to be in the same situation as NetBSD and just need a driver to register the NuBus-based OHCI controller to the OHCI driver... Unfortunately I don't have a NuBus PowerMac (and my PCI one
is quite dead) or any knowledge of 9.1 internals.
In all cases - this would be the kind of software written as a proof-of-feasibility; there won't by any money in it (especially if you need hundreds of euros worth of custom hardware just so you can plug an USB mouse on your PowerMac 7100 ...).