Checked the Catholic Canon law and there's NOTHING that says how we have to dress for Mass.
Despite the old priests railing on about flip-flops and shorts, no one seems to listen.
And in the Middle Ages, the priests used their high perch to look down at plunging necklines and call cleavage the "gates of hell".
Lately, I've noticed these wacky guys in Church who dress up like the 3 Musketeers. Boots and all. And I've seen all kinds of sisters dressed up in habits, until it becomes impossible to tell which order they are (and there are so many!)
-Dress up in your new "order" as a sister, an "order" you just made up
-This allows for all sorts of imaginative creativity
-And this can be male or female, since in Catholicism, everyone wears dresses
-When anyone asks you, act like you don't speak their language or even better pretend to be deaf
-For those with big budgets, go to the religious supply store and buy vestments
-For those with smaller budgets, make your own
-Don't process down the aisle as that might be trespassing and they might call the police (which are the real determiners of Catholic theology these days...the bishops have left everything up to law enforcement, since they've shrugged off all responsibility)
-Just sit in the pews (see above for different language or being deaf)
-For communion, you're probably out of luck in suburban parishes, but inner city parishes are used to crazy street people showing up, so you're much more likely to score there
-For women, this is a little dicier, as the hostility will be palpable and you might have some little old grandma cussing you out
-Bonus round: There is so little communication in the Catholic Church, that you could probably pretend to be clergy from far away. You might even be able to process up the aisle and help "celebrate" Mass. Just wear a Roman collar and black pants and mention it to the worship coordinator. They'll probably let you right in and GIVE you vestments! Score!
(Warning to Women: I had a good friend who worked with a co-worker who was a "former" Episcopal priest. That turned out not to be true. The co-worker had been a "priest" in pretend Catholic church. The co-worker not only kept his previous vestments in their offices, but told parishioners he was a priest and actually heard confessions. But no one believed her, because she was a woman and he got away with it being a man. Please realize that being female will automatically mean that you're at a disadvantage. But that makes DIY Catholicism just that much more challenging and fun!)
Good luck! If you pull this off, please post pictures!!!