Correction on the above -- I suggest adding the Assertion in the VSI Selection Step not Responder Step.
In fairness, I had never tried it and Francis decided to be my guinea pig.
The shortcut way that might do the same -- now that Francis has proved this out would be a simple
return ( testExec.getStateObject( "lisa.vse.matched.transaction" ) == null );
I normally use the approach of setting the META response and asserting on it in the VSI Selection Step.
Like you, Mike, I have never attempted a Match Script approach so unclear as to how that would work.