I don’t have the wrx wagon, I got the wrx sedan but I know what you are talking about. I had a ford explorer before this car and I was going to install some speakers in the back gate (or in your case a hatch but similar) but didn’t because the ford was broken into 3 times to get to my stereo so I decided to sell it. The only real constraint that I could think of would be size. I looked at the wagon and the hatch in the back. It didn’t look very deep to me so before you go out and buy stuff I would take it apart and do some measurements. Then you got to take into consideration about the hatch it's self. Are you going to get a new panel that will have slits for the speakers or are you going to try to make them your self? My ford would of been easy to do because it already had slits for speakers and had a lot of room. If I remember correctly I don’t think the panel for the hatch has slits in it so this adds some difficulty to it.
IMO, I don’t think it is really worth it. If you ever got subs in the back they would just pound the F out of the hatch and your speakers, which in the long run could cause some problems.