Well, you could always try phantom center, ie. no actual center speaker (w/ the center channel remixed into the front L/R), if the front speakers are good enough and won't be set all that far apart (probably no farther apart than their distance to the main seating area) -- I assume your AVR has...
You could try it and see for yourself, but just don't get your hopes up too high, especially since your entire current 5.1 system is already firing from the ceiling(!), if I understand correctly. Is your subwoofer actually also in the ceiling???
If you can add proper audience level speakers...