Yes. Part of the motivation for RISC-V was to enable designers to do one-off experiments like this or small production runs with custom ISAs based on RISC-V, without having to worry about licenses or intellectual property issues. RISC-V even leaves some op codes uncommitted so designers can add their own custom instructions. Dave Patterson (RISC-V designer and Turing award winner) explained this at a talk I attended a few years ago.