This is a nice coincidence! Later today, we were planning to create a GitHub repository for StableHLO - a stable version of MHLO (and CHLO).
At Google, we have a team staffed to contribute to e.g. a spec, a reference implementation and a test suite, as well as new feature development (dynamism, quantization and sparsity are the big ones that come to mind, and I see that you mentioned them as well above).
An open question is the governance model - let’s figure it out together. At the moment, MHLO is pretty much Google-driven, but this is something that we want to change with StableHLO.