I had a toy implementation based on DynamicHMC, but obtaining the Hessian was the bottleneck (I was using ForwardDiff). The current refactor of DynamicHMC & related libraries will keep the possibility of adding an RHMC kernel open, but the idea will become practical when we can do Hessians fast, possibly with something like Zygote. The SoftAbs metric itself is relatively easy to implement.