Hi there, 

>> I've only ever seen this on a HP Integral PC, the ROM module sat in
>> some weird socket on the back of the unit.
>
>Hmmm. I better start looking, it might be a fun thing to have :)
well, the Integral PC is no series 300, merely a 2something so NetBSD is not an option here, even if you manage to get a model with more than the standard 1MB, it has an integrated Inkjet printer, 
some HPIB interface, a monochrome LCD and a small HIL Keyboard ( without numberblock ).

have fun
Michael