....so I have to thank my bosses boss for pointing this one out (and I guess this means I've also found out who the third reader of this blog is :) ), but there's also a little known method, SetConfigurationValue, in the Virtual Server 2005 R2 COM API. This is another way of solving yesterday's problem of changing the BIOS setting in a Virtual Server Machine Configuration (VMC) file without rolling something using your own XML parsing. Of course, you can use the same API to change lots of other settings apart from the BIOS, and the same warning about lack-of-support in changing the BIOS settings this way applies.
Here's the link Mike pointed me to: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/msvs/msvs/ivmvirtualmachine_setconfigurationvalue.asp
Not much help for changing boot priority to PXE. You still have to know the right hex-string and this one changes i.e. if you add a network card.
Yes, appreciate that - changing the BIOS hex string is the unsupported part :( I was just posting that rather than rolling your own XML parser as indicated in the linked posting, there is an API to update the config file in a supported way, just not that part of the config file.