The lack of feedback here is amazing. Maybe because it's a complex matter and hard to imagine all its consequences without actually using it in production instead of fooling around with test scenes. Tbh, I haven't played around much with mask propagation, Corona v6 generally (was hoping for some improvement relating to opacity handling which I would need most) but let's see:
1) I remember using the "visible to mask" checkbox repeatedly as some kind of last resort in some cases without being able to provide them. Misuse like
alpha stencil seems to work in v6 as well (with propagate mask -> Always instead of invisible mask). So a tentative "no" here.
2) The consequentially correct result would be black. It's not the object we see in beauty, so why should we see its mask? It should be handled like alpha is treated in such a case.
3) The idea is tempting and may be somewhat realistic, but I vote less tentative for solid here. It may only lead to confusion - and to unusable masks due to that noise from what I've seen. Most puzzling result is the mask "mix" you get using high(er) glossiness reflection with low diffuse amount. No one will like or understand that I think. And rendering 120 passes for a usable mask may be far away from production reality.
I may be wrong with all above! :) Curious if others can contribute their thoughts,
Good Luck