Is there a testing framework that would be appropriate for IU overlap testing? i.e. is there another existing test that does something similar?

Blink itself has several layout tests, so it is possibe. Having said that, I don't know much about it, especially on whether this can be used easily outside of the Blink codebase.

@tommycli has mentioned in the past that such tests (pixel fidelity, screenshot diffing) are fairly hard and high maintenance.

Yes... those are high maintainence, but in my use case, have caught about 2-3 attempted regressions by other people.

In your guys' case, maybe consider keeping tests that query the pixel location / sizes of elements without taking actual screenshots.