I was thinking about this last night. In terms of the code involved, the upgrade should be extremely simple: open the BIOS for writing (probably one assembly command), take this block of code and write it to such and such address, and lock the BIOS. Since it is probably written in assembly, Wine would only need to interpret the .exe structure. I'm thinking this should be perfectly safe.
On the other hand, I am right now in a situation where I am completely dependent on my lappy and have no backup. A few day ago I was even worried about the kernel upgrade that got released. I think for the time being I will heed the warning in your sig.