You still have a few options available, though admittedly the easiest avenues are not available.
The first thing to check for is a clear CMOS jumper on the motherboard. Consult the manual (if available) or check the board itself - if you have one of these, simply bridge the pins, boot the machine up and the CMOS will be restored to factory defaults.
There are a limited number of "BIOS password readers" available, which may in some cases be either to read or remove the password. However, many of these utilities are strictly unofficial, and should be used with care and at your own risk.
You could try sending your BIOS/CMOS chip to a specialist for flashing, though that is a time consuming option and may not be worthwhile given the age of these boards.
Finally, there is a high-risk option that you use strictly at your own risk. You mentioned you have several of these boards - all you need is one with a non password protected BIOS and you can hot flash your corrupt BIOS.
Here is what you need to do:
- Boot a machine up using a non password protected BIOS.
- Install a floppy disk drive and ensure write the appropriate flash utility to disk.
- Reboot with the flash utility floppy in the drive.
- Swap the working BIOS with a non-working BIOS chip whilst the machine is running. Be very careful not to touch anything else and for goodness sake stay well clear of the power supply unit.
- Proceed to flash the BIOS using the utility as normal.
- If all is well, remove the floppy and reboot - the BIOS should now be reset and working correctly.
Now as I stated, this is a risky procedure and you use it at your own risk. However, this is a recognised method and usually used as a last resort. If in doubt, Google "hot flash +BIOS" and you will be given much more information on the subject.
I hope that helps, let us know how you get on.