I've got a USB disk box with an IDE-to-USB adapter.  Unfortunately, it seems
not to probe as a disk device, instead getting picked up by the ugen handler:

ugen0 at uhub0 port 2
ugen0: OnSpec Electronic Inc. USB Disk, rev 1.00/0.08, addr 2

Is anyone by chance already looking at supporting this?  If not, any
words of advice on how to get it working myself?

Hmm, I don't see any indication on their web site that they document
this adapter, and there doesn't seem to be a linux driver...  Would
the kernel have recognized it right away if it implemented a standard
USB Mass storage protocol?