[tim chapman]

Does anybody use - or would advise - to lower the clubshaft plane at the top of the swing as the downswing starts - a fall down, a little clubhead loop - in order to get a shallower and more in-to-out swing path into the ball?

i think you are talking about shifting from say the turned shoulder plane onto or more towards the elbow plane, this is a legit TGM move i believe