Don't go right to the floor on squats. Stop when your thighs are parallel to the floor. That should take the ass out of the lift somewhat, and might help your rear from blowing up.
Ha ha ha yeah I know... my pants are starting to get uncomfortable and my legs rubbed together when I walked last week. I'm laying off all upper leg work (even quads) for a few weeks, to focus on upper body, calves and cardio.
I'm genetically predisposed to store a high proportion of fat on my ass. Additionally, squts tend to pack the mass on to it quite easily. I know how you feel, but its really not that much of a big deal.
I haven't stopped training certain ways because of how my body decides to adapt to the exercises that best suit what I'm trying to achieve (more often than not, sporting performance, but at the moment - aesthetics). Hack squats are a good option, but depending on your goals, be careful that you're not selling yourself short in other areas...