I’ve done this before – I had to hold my nose while I did it, but I did it. Pragmatism beats dogmatism every time. Of course, if there is a nice way you can refactor to avoid it, that would be great.
Basically I had a “UnitTestDetector” class which checked whether the NUnit framework assembly was loaded in the current AppDomain. It only needed to do this once, then cache the result. Ugly, but simple and effective.