There are a variety of x86 products with Coreboot support, if what you are looking for is firmware openness. If what you are looking for is PCB design openness, the options are much fewer, but at that point you are probably optimizing for an overly niche objective.

> Part of the point of this for me is to see what's possible with open hardware (down to chip level at least)

I appreciate the idea, but this is essentially saying "this project will prioritize a specific choice of one (core) piece of hardware to the detriment of everything else, users included". Approximately none of your potential users are going to benefit from the "openness" of the SBC versus that of a more broadly-supported platform (I say "openness" because the reality of SBCs is that actually finding a usefully performant one that is completely blob-free is almost impossible). Open hardware means very little if it isn't running an upstream kernel and userland.

Framework? Maybe?

You probably aren't using some weird SOM, though. There is a bit of an unstated exception of "unless said SBC/SOM has specific hardware that is necessary/particularly valuable for your product/project". For example, if you need GMSL you are probably not going to be picking Intel, even though ADL-N and the bigger processors support MIPI, simply because no one else does and the documentation/support for it is basically nonexistent. Designs with closely-coupled A/M/R cores, or CPU/MCU/FPGA hybrids like Zynq would be others.

But generally projects which are choosing some random SBC aren't using any of these features, and are just suffering the pain/imposing it on their users for no good reason.