Are you looking for a legal loophole to “dip” below MDA, just because you can do it on a PA? Or do you want to look at it practically from an approach designer’s perspective and the application of approach design criteria? The “dip below” is part of the design consideration in a PA and is protected. Not so for a CFDA flown to an MDA. Helicopter/airplane makes no practical difference. If the regulators decide all non-precision approaches shall be designed in a way that the “dip below” when flown CFDA will be protected, then designers will simply raise the MDA ((now a DA) the appropriate amount. Which would you rather have?