I’d like to hook into the venv setup step wherever it is used (pants run/lint/fmt etc.) so that I can validate connection to private package repos and provide resolution info if needed. I’d like to avoid doing this if a valid (per requirements) environment has already been created and hence no download is required from these private repos. Where should I be looking to hook into to do this with plugin?

I'm not super-familiar with the internals of the Python backend, but I think for the most part there is no explicit venv at all. Mostly everything goes via Pex -- so I'd look at

pex_requirements.py

, maybe

pex.py

. Those are at least points to start digging.