I agree with you for the most part, however, even though both 2,4,5-trimethoxy-PEA and 2-methoxy-4,5-methylenedioxy-PEA both have vicinal oxygen atoms, this does not mean they will share the same metabolic fate.
Think about 2,3,4-trimethoxyamphetamine, the only TMA compound which is considered to be inactive. However, the same exact substitution pattern is shared by MMDA-3a and MMDA-3b which are both active at less than 100mg.
This makes me think that perhaps the conformationally restricted methylenedioxy ring is more resistant than the corresponding free methoxy derivatives. Consider Nichols' work with benzofuran derivatives based on the DOx-series which has led to the discovery of some of the most potent phenethylamines known.
Of course this discussion is purely theoretical, and the effects a compound will exert in vivo is nearly impossible to predict. Certainly the unique action of MDMA could never have been expected based on the results of N-methylation of the other psychedelic amphetamines.
It also has as much, if not more, to do with the compound's interaction with 5-HT2x receptors than it's susceptibility to metabolism. Besides, MAO can be taken care of w/a MAO inhibitor (orally) or simply administering the compound thru a different route (i.e. smoking the freebase.), this requirement hasn't diminished DMT's popularity that much is for sure.
I just feel this would be an interesting compound to explore, considering no one has (to my knowledge) done so.