This isn't in our pipeline. Ml-Agents will support any physics engines that the Unity engine itself supports.
I understand, but the Google Brax Physics system is aimed at RL speed. I don't think that the unity engine will support it on its own, even though it claims to provide significant performance improvements for RL.
Totally agree, faster physics would be great! New physics engines though are quite out of the scope of the ML-Agents product. Unity is working on a new faster physics engine for the DOTS platform, so if your game is using DOTS you can use it. (currently you'd have to use Hybrid to use ML-Agents with DOTS).