I read somewhere RealVNC had an option that included staying connected during BIOS boot.
I think the Supermicro technology you are referring to is IPMI (see:
Intelligent Platform Management Interface - Wikipedia), but lots of systems also have Intel vPro AMT (see:
Intel Active Management Technology - Wikipedia). Both are built-into-the-motherboard low-level access interface for remote management of a PC.
I recall reading about security holes in Intel's vPro technology (some are referenced wikipedia) and IPMI has had it's share of security vulnerabilities as well. IPMI is even called "the most dangerous protocol you've never heard of" by IT World (see:
IPMI: The most dangerous protocol you've never heard of)
It's hard if you cannot get easy physical access to a machine, but I suppose some combination of IP KVM or autoboot with a network addessable outlet probably could do the trick if you can't make TeamViewer, VNC or Microsofts built-in RDP to work for you.
