My understanding is that rectal administration takes less actives to work, it's a mucosal pathway, and things that work sublingually usually work better (with low doses) in same quantity than oral, usually not as long.
Digesting seems to "waste" some material.
Of course, it all depends of the substance, but it does make sense for some (or most ) of them.
Is mecaline active sublingually ?
I would say yes since the other day, SWIM was scratching some mesc acetate and was licking his fingers each time he would have some of it in contact with them (using a razor blade, the acetate was somehow climbing up his fingers). He smoked some changa (so there maybe potentiation as well) and felt a very nice effect all evening long (afterglow of harmalas extended with some vibrating mescaline energy). Note that the dose was very low and spread along his mouth mucosa, but he felt it more than a three fold dose oral.
Smell like tea n,n spirit !
Toke the toke, and walk the walk !