After watching this morning's episode, I had a little epiphany.
You know the little fake, washable tattoos they make for kids? Well, how about fake, washable Cutie Mark tattoos? They're just about the same; nothing like the real thing and completely harmless. They would not give the users any special talents, and are there just for show.
I could imagine a little part in an episode (or, perhaps, a full episode) where the CMC "get" their Cutie Marks (a.k.a. the fake tattoos) and, because people in Ponyville conveniently never ask for validation (perhaps they've never seen CM Tattoos before), everyone congratulates them on finally getting their Marks. (Even maybe Diamond Tiara and/or Silver Spoon. Probably not gonna happen because of their characters, but it's just a thought.) But of course, the CMC are found out as the tattoos, due to a mishap involving water or something, get washed off, exposing their still blank flanks. End of episode.
Then again, maybe I'm just crazy. Yeah, that's probably it.