We ended up contacting Tyan Support who supplied a utility called kcsflash.exe which was able to change the MAC address.

Was a little tricky, but worked as follows:
1) Reflashed system bios with latest, reboot, save bios
2) reflash AST2050 with kcsflash using NoConfig Y options. full power off cycle.
3) kcsflash MAC XX:XX…
And it worked :-)

I have dumped a copy of kcsflash here: http://www.firefishy.com/util/tyan-ast2050/KCSFlash.exe
USE AT OWN RISK.

/ Grant

I can add that the (very hard to find) uh8.exe DOS tool also makes it look like it can change the MAC, but it really doesn’t.