If I understand you correctly: You want to be able to record one computer with another one on a system level (the BIOS-party that comes before any operating system is loaded). Although this is not Linux specific: your best bet is a video capture card as you've suggested already. Anything else would depend on your bios supporting remote access which is not exactly the same (my server bios for example can expose a website where I then can configure it from within a browser. The problem with video capture is that you'd still have two controls: one for the client and one for the host. Depending on what your final result should be it could be actually easier and cheaper to just get a stand for a smartphone and record it from there and then crop it precisely. You then have to only worry about light reflecting.