You can make a Giratina-O with Powersaves, using the form change code. I've not tried it myself, but I've heard from someone else that if you give it the Griseous Orb while it's in the Origin form, and then take it away, it reverts back as you'd expect.
You can see videos of permanent megas on YouTube. I'm very surprised that mega evolutions don't revert after back after the battle ends, but like Kaphotics said they revert if you keep them in your party.
I've not seen anything on this so far, but what happens if you put them in Pokémon Amie? And do they do things when you tap them on the status screen? Permanent Pirouette Meloetta crashes Amie, but has a unique set of animations (like doing a pirouette) when you tap it on the status screen, which is weird for something which technically shouldn't exist.