Hi Elmar:
I was thinking more in the "standalone boot program" approach: A test plpbt.bin program with just the Cardbus enabler, that can boot from a standard floppy/cdrom, initializes the ethernet pccard and then load GPXE and passes the control to it. This should work because PLoP already can boot directly linux kernels like syslinux, and gPXE have the option to be compiled intro a standard boot image that can be loaded with a "kernel" option, just like memdisk/memtest programs.
I'm saying this because the ROM approach in laptops is difficult... not only for you, the dev, but for users as well, because laptop ROMs are non standard and really difficult to modify, and pccards are external devices... so for test purposes, i think that a cardbus enabler floppy/cd boot version is better ;-).
As for gPXE getting the required support, well... i lost my hopes in the gPXE/iPXE team, they stay saying that is a difficult thing (you showed that isn't for a talented dev ;-)) and they are focused in other things...
Greetings