The question is simple but the answer is not and that probably explains why people aren't lining up to offer an answer.
As far as I know, the only practical and legal solution is to contract with the BIOS manufacturer for a custom build of the BIOS.
But that also means that future BIOS upgrades will be a problem and you will need a separate build for each motherboard you use. It sounds the making of a support nightmare to me.
A splash screen when Windows starts is a far more practical solution, or even a splash screen called by the boot software.
I know this isn't the answer you wanted, but it's the best I can offer.
paul