I've seen speakers there.
The only bad part about the speakers being there, that I see, is;
1. They could act as a catch all for anything that falls back there
2. the sound will be muffled if you carry anything back there.
3. they are aimed up at the roof and not where the sound will reflect well into the listening area.(but GT are small enough, this might not matter.
It would be a great place to put a sub-woofer in. Even with stuff covering, it would kick out some great bass. Might even help it. I talked with a sound guy that told me some of the theaters will put subwoofers in boxes, aimed inward, with styrafoam peanuts in it to kick out more bass.
You could try running your wires and hooking up the speakers temporarily to see how they sound. You could try them in a couple of spots and angles to see if you get the desired sound. You might even want to build a small support surface to angle them forward a bit.
I mounted a removable sound box in the spare tire carrier with enough wire that it can be removed and put on the rear spoiler for tunes outside. It has Subs, and tweeters in it for full sound.
Good luck.