I'm guessing you mainly think of planning and maybe review when you say that Opus and GPT vary in their output. This is more of a qualitative judgment because it's difficult to write a planning benchmark. If you wanted planning to pass between both of these generalist models, role-model supports this via the pi-role-model extension for Pi that lets the coding agent send request metadata, define routing strategy to use and even model ID.

To implement it, you would make a rule for Pi to do another pass for any coder.planning task that it sends to the role-model router.

These types of personalizations are suited for implementation in the consumer application layer instead of in the router in my opinion.