I carry a P3AT in a Desantis nemesis cargo holster daily, and have for about 6 months. There are a couple things I have found to consider that are along the same lines as others have mentioned.
First, the holster, like I mentioned, is a Desantis Nemesis cargo holster
(Link here) which instead of having the normal hook at the bottom, extends out with an extra magazine pouch which keeps the gun from tipping over in your pocket.
Second, I try to pay attention to the pockets when I'm buying pants. I like the Wrangle Originals. I can't see any other identification on them, but they have smaller pleats in the pockets so they don't expand out very much. I have found they keep things in place nicely. My work has me moving around, crouching a lot, sitting while driving from stop to stop, and everything stays in place just fine.
Finally, as far as accessibility goes, with the Wrangler pants at least, they have velcro securing the pocket, so it opens easily. Buttons are definitely not ideal, but I have a couple pairs of pants with button close pockets that work ok, I guess. Just like any draw, it takes some practice. Yes, it is slower that from an IWB or OWB solution, but for me, it works fine. If all else fails, I figure the only time I would be trying to get to it would be in the type of situation where I wouldn't care if I had to rip a couple buttons off the pockets in order to get to it.
I MUCH prefer to carry IWB, but since my work attire and activities won't let me carry any other way, this is what I do. Obviously not for everyone, but I think it's a valid form of carry. Better than nothing...