I can't see any reason why you couldn't do what you're proposing. I've been fiddling with ethernet on little micros lately using the microchip ENC28J60 chip. Futurlec have a little board with all the necessary bits and pieces to make this chip work then all you need is four signals from the 8051 to talk to it. The code I've been fiddling with is this:
http://tuxgraphics.org/electroni...rver.shtml

It's in 'c' and for a AVR, but that shouldn't stop you writing it in assembler - the code is pretty straightforward and you can cull out the bits you don't need.