This would be welcome also from the RISC-V Foundation's side (and some other people who I know might be interested). I am on the marketing committee and we're always happy to see new activity, the more choice of OS, the better.
Also in my role in Antmicro, we are building workshop badges based on SiFive E310 for the next RISC-V workshop in the Bay Area so it'd be cool to have the RIOT port running there too (currently we're using Zephyr).
If you do manage to launch that effort, be sure to remind us to put you in the software status list: https://github.com/riscv/riscv-wiki/wiki/RISC-V-Software-Status
Best regards, Michael