The same question was asked no too long ago. It can certainly be done... 15" drivers facing forward would work well. I used a pair of 18" drivers, down-firing when I built my stage for my theater. (would have been too high for them to be front facing)
Check out this thread for ideas:
http://www.hometheatershack.com/foru...ont-stage.html
You can see my proscenium subwoofer here:
http://www.hometheatershack.com/foru...html#post64660
Joe L.