This started last summer and a number of interesting designs have been produced, pretty much entirely with free, open source tools.
Join the slack and ask around for projects that need help if you want to dip your toes in this sort of work! https://invite.skywater.tools/
That said, one could probably use the free (but not open source) Vivado HL WebPACK to create and test the RTL design and then follow along with the Skywater tools. But that way, the way towards the FPGA would still be closed source and only the path from FPGA on to ASIC would be open.
Further down the rabbit hole, I read about https://github.com/icebreaker-fpga/icebreaker which appears to be a tiny Teensy-compatible open source FPGA board with accompanying open source tools. Yay!
Thank you very much for bringing all of this to my attention :)
I'm interested, but almost entirely on the analog side -- I'd love to design a NLTL, VCO, or distributed amplifier. I have the utmost respect for the crowd looking to scale RISC-V cores, but it's just not my scene.
EDIT: I took a look at https://github.com/yrrapt/caravel_amsat_txrx_ic_mpw2 and there are clearly some analog projects in there. Evidently I should have hustled harder!