Does there exist an RPI clone built around a SoC that is suitably open?
But for real openness, nothing beats an older x86 PC.
Rockchip have a reputation of fully working with the open source community, though I can't personally say how well. Most other SoC vendors only do enough to get android going.
However, I was referring to the availability of documentation which for these Chinese SoCs is either officially released or soon leaked.
I'm not taking the "security" bait at all, especially when we're comparing a PC to an even less-explored platform which also requires binary blobs.
(but I'll look into coreboot. I'd get a kick out of trying it.)