http://sparc.org/technical-documents/
One embedded implementation with a ton of supporting I.P. is GPL'd, FPGA-proven, ASIC-proven, and rad-hard in some verisons:
http://www.gaisler.com/index.php/downloads/leongrlib?task=vi...
And then there's Oracle, their badass chips, and their evil ass lawyers. We can stay away from all that. SPARC is better and safer than Oracle but very importantly SPARC != Oracle.
Best way to deal with them is to straight-up license their tech for a Pi- or router-style board. Then fab, assemble, and sell that joker. That gets the ecosystem going. CompSci people doing CPU's or RISC-V work can keep building reusable components both can use. Then we just pay for the integrations.
I suppose cathedral vs bazaar doesn't affect that at all, but experience has ingrained in me "source tarball ==> won't easily build" biases.