Midbass behind you should theoretically work, with a few caveats;
1) You maintain accurate time arrival of the midbass
2) You adequately keep high frequency content out of the driver, so you need to really control your crossover point/slope & harmonics
3) You eliminate any rattles/resonances that identify the midbass location
4) You keep the drivers as wide as possible
Just off hand, RC/SW Buick Grand National & Harry Kimura's(sp?) Acura used rear mounted midbass drivers successfully in competition, when there really was competition.
If ECA wasn't dead, Werewolf had a great thread on this subject.