Insurance does cover negligence so I think it would not be void just because the aircraft departs over MTOW. Also, passengers can and DO lie about their weight.
Any views from LAWYERS?
But to be practical I am sure the majority of PA28s flying with four people are in fact over MTOW. I used to fly an Archer and 4-up one would have so little fuel one could not go anywhere useful, with a real weather diversion reserve. With a longer runway, and an appropriately higher Vr, it works, up to a point.
Most 4-seaters cannot carry 4 average men (or, these days, four average women
) plus useful fuel. A decent tourer like a TB20 can but a PA28 can't.