There a few incomplete ports of Jikes RVM for other architectures out there, most notably 32-bit ARM and RISC-V (not public). We should try to make enough progress on JMM so that porters can properly implement JMM for new architectures. For that purpose, support for prototype would be probably sufficient.
Additionally, we also need to have enough JMM support so that the system is stable enough when using OpenJDK as the class library. This will also require work on JMM for the opt compiler.
It's possible that we'll focus on 64-bit and leave 32-bit JMM issues to porters.