I'm pretty sure Ebookman had native USB, the cradles were for serial connection.
(I think it needed special drivers for the PC to see it, though.)
Might be able to sneak in an Ebookwise 1150 since it needs special drivers to work with PCs. They could try plugging it and see it won't connect.
(Since it relies mostly on the SmartMedia card it might even be possible to fill in the ports with epoxy and lose no meaningful functionality.)
Interesting problem, really.
Thing is you have to go pretty far back, techwise, to get something *without* native USB and at that point there might not be many ways to read ebooks on it.
The one device I *know* for sure meets those requirements and is good enough to read ebooks on is the old Jornadas 'cause I used one for several years.