What if we just make mtr to do `exit 2` if it exists because of --max-test-fail ?

I'm afraid that won't help me in any way.
In such case, still, no matter how many tests fail (if >0), the testsuite will fail. With different exit code maybe, but it will fail.

I'm looking for an option which would say "If only <N tests failed, return success anyway".

That would spare me of rebuilds beacuse on or two unstable tests failed.