This idea is great, I've always wanted something small and more practical than a huge 28 lbs case.
but 386sx is painful. I believe the sweet spot would be something like a 166 mmx (at least for my needs).
I once envisioned a PCI-Express card that contained a super socket 7 era chipset, and a processor, as long as the ram, sound and video card were emulated under software, it would greatly reduce the PCEm/86box requirements.
Well as many people may already know, I have had ideas that are, like yours, a large project.
But to try to break it down with your ideas in mind - you would still need your team of programmers to use opensource BIOS. Otherwise you get a machine that has proprietary BIOS. Either way it is a form of hardware drivers, mostly for the chipset. For the P166 w/ MMX and socket 7 (quoting retro web):
The Intel 430TX PCIset (430TX) consists of the 82439TX System Controller (MTXC) and the 82371AB PCI ISA IDE Xcelerator (PIIX4).
There's a level of magnitude difference in hardware level programming. You can see this by consulting data sheets for the above mentioned components. And then you need to have parts under test to get the proper input and output to various components, namely ASICs et al.
This is why I like the original IBM AT era so much because I now have this huge collection of books from the market in those days. Costed me hundreds of dollars in out of print books.
With this time frame I can learn the basics of hardware without the "microprocessor course" feel where it's
humbling to ask the instructor about embedded systems. You might know what I mean by this if you saw youtube videos. It's stuff you don't get to until much later in college.
Would rather do the real thing. Which lately, speaking of my 486 & 586 builds, has been more of a jumping off point.