I've successfully built it, a few weeks ago... if you're interested I can put my pcb-layout on a webserver.

For 89x2051 I use an adapter which I put into the 40Pin-Socket and two addisional wires for the pins which the 89x2051 has not in common with the 89x52.
But be warned ;-) there're about 35 wirebridges (word?, it's "Drahtbrücken" in German), .
I've not yet tested whether it works for 89x52, but I've you're interested, I could do that.

I've also ported the 89x2051 -flash-programm to Linux, the main thing was replacing some function calls to clear the screen and replacing the timer-functions and hardware-io.