Even some ISA cards can be 386-specific: I know my ATI Graphics Ultra
Pro (ISA) card manual at least stated that you need a 386-class machine
to use it (it _may_ be that this was due to external drivers, but I
wouldn't be surprised if they use 32-bit operations even in the BIOS).
Still, just emulating a real-mode i386 is trivial, at least compared to
emulating protected mode and paging.. There are various x86-emulators
around (although the one I remember off-hand had some restrictions on
use), and extending them accept 32-bit data extensions might not be too
bad.
It wouldn't need to be very fast, so a stupid and straightforward
approach is pretty ok. You'll be shocked to hear that some of this is
actually done in the linux i387-emulator already (it does all the memory
access emulation already - looking up the mod/rm stuff and handling
different segments for v86 mode, for example).
The i387-emulator obviously won't do any of the actual integer
instructions, but it _could_ be used as a starting point. After all,
what it does is essentially emulate a subset of a ix86 already..
(although you may not actually want to have all the baggage that the
math-emulator has: it needs to be much more advanced than your average
BIOS emulator in some respects).
Linus