more tests, still no luck. I had access to a hardware flash/EEPROM programmer today and flashed bitworks/ims/linuxbios.rom (256K this time, last time it was 512K which obviously cannot work). The Windows-Software for the programmer supported the chip (W49F002U) and reported that flashing worked fine (verify succeeded, too).
Now, I tried to boot from that chip but cannot get serial output, it seems (115200 Baud, 8N1, ttyS0).
I also tried to set the "reset BIOS" jumper on the board before running "./flashrom -V", but it seems that doesn't help either. I'm slowly running out of ideas.
Hm, say I accidentally killed the chips (ESD) somehow - would the result be 0xFF's returned as vendor/device ID?