AFAIK Machynlleth guards sign SHR-BHI, but don't currently sign anything other than 158's.
The unit diagrams for BHI services all include work on the Cambrian too.
The units themselves are currently cleared for ECS moves on the route, the current test runs are - I think - for things like stopping positions and ASDO configuration.
Given the first two points above, I'd guess that it's unlikely (although certainly not impossible) to see 197s in passenger service on the line until the ETCS units start entering service on the Cambrian.