Due to the speed limitation of the Serial interface, I wonder if you could run the 3com card with low enough resources to do the emulation. Not real networking, I am thinking more of embedding requests into raw ethernet packets with a direct ethernet link to a Raspberry Pi, and some stuff on the RPi?
Alternatively the Printer port supported 8bit parallel transfers are was much much faster. Potentially a USB -> Parallel interface on the Pi. I remember using a Parallel to Parallel cable between 2 486s with the lap link software and getting MUCH faster then serial connection.
Also, it's pretty easy to make a wire-wrapped ISA-bus card.. I would try to do this. One 22v8 GAL for the address decoder :-) I would make a some kind of FIFO interface- don't bother with DMA, it's very slow.
Alternatively the Printer port supported 8bit parallel transfers are was much much faster. Potentially a USB -> Parallel interface on the Pi. I remember using a Parallel to Parallel cable between 2 486s with the lap link software and getting MUCH faster then serial connection.
Pretty cool.