You could go as the Genie master and she your Dream of Jeanie harem girl.

I always go as a belly dancer because I love those outfits, lots of flowing veils and sexy looks from beneath them  Very sexy and your ATF will look great gyrating on the dance floor!

a married couple attended: he wore a 'standard' striped prisoner outfit, handcuffed and leg cuffed. SHE wore heels, skin tight black leather pants, a blue silk police shirt with the top buttons open over a black push-up bra, and carried a dildo blackjack. (she's about 34C-26-34, by the way) every other guy at the party spent the entire night pretending (to his own date) that he did not notice her. it was a riot.

he's a good friend of mine. he told me that they went home at about 2am and had sex for about 6 hours; showered, went out to breakfast, then came home and went back to bed til about 7pm. he said it was the best sex he ever had, and that they made a deal that she would take him prisoner again on his birthday

Won't cost you a cent & sure to be popular with the other guests.

How about an Electrical Plug and Wall Outlet

Seems appropriate.  She should go as the plug.

Go as her pimp; have her in "street" clothes.

Want to hide something?

Put it in plain sight.