The Olimex ESP32-C3 module has a risc-v core, sure, but it's roughly equivalent to an ARM Cortex-M3. The HW platforms targeted by Plan 9 and its tools are much more heavy weight. I don't believe Plan 9 would be a good starting point for any ESP32-C3 development. If the Olimex guys are looking for a monitor, then umon might be a good place to start.
The code is modular, clean, already supports about a dozen architectures, and should port to the ESP32-C3 without too much work. And it shouldn't be difficult to add risc-v support to its built-in 'dis' disassembler.
Just my $0.02 ;)