I have been trying a lot on capturing the BIOS. Until now I didn't get to a resonable solution. I have tried capturing the BIOS by using 2 PCs, one of them with a S-Vídeo output and the other with a S-Vídeo input.
Since S-Vídeo is an old tecnology it only alows for resolutions up to 720x480 which is ok to capture the BIOS because the resolution isn't so high anyway since it is not using the video drivers of the video card jet, just a standard drivers.
If you only want to capture BIOS s-video is fine but I wanted also to capture other things like booting up from linux and live CDs \ flash like G-parted, norton ghost and Windows Repair disc etc. So I bought a HD capture card (Hauppauge colossus) and used a thid party software (nextpvr) to capture the screen. This did only work partially for me because there was always a delay between what was going on with computer to be captured and what was showing on the screen.
I used an HDMI splitter to put 1 output of the PC to the monitor and another to the capture card.
What I didn't try for now: I know that vmware workstation has the ability to virtualize a real machine (I don't know how this works yet) maybe it also vitualizes the BIOs somehow.