Hi Mascara^Snake,
I have an idea or two for you, but I have no idea what kind of belt you are wearing, with what type of rear, so hard to say how comfortable the plugs might be?!?
The first toy I thought of when you brought it up, was the "Pro Touch Plug" by Tantus. You can see it here:
http://www.edenfantasys.com/vibrators/anal-vibrators/pro-touch-plug It looks like it is manufactured specifically for that purpose. But please note, that I have NO experience with this toy. I have never tried it, nor do I know how well it would work, or if it would twist on you while wearing it. You can red reviews from actual users on that site I linked to though.
he second thought that I had was the njoy pfun plug. The link is here:
http://www.edenfantasys.com/anal-toys/anal-plugs/pfun-plug Njoy makes PHENOMENAL plugs. I have one of their "pure" plugs, and it's amazing. I've wanted to try the pfun plug for quite sometime, but there's been too many other things on the wishlist so far. Their plugs are beyond comfortable, yet weighty enough to remind you it's there. You can't feel the handle while sitting, and it'll prevent the toy from twisting on you. It is designed to work your prostate while wearing it, and is rigid so I can see it easily providing the pressure and stimulation needed for a milking. The only thing I don't know, again, is how it would work with your belt, depending on what you are wearing.
Good luck in your search!