Can we then say that failure of an item requiring/resulting in a limitation restriction (i.e. runway length) can therefore be ignored as well?
Once you have started taxiing, the MEL no longer applies. Common sense, however, should.
Exactly - if a performance-limiting item fails after engine start but before TO, one would be daft to disregard the FAA: "For MEL purposes, 'Dispatch' is defined as the advancement of throttles for the purpose of taking off (takeoff event). "