I'd look into a wide dispersion Paradigm center channel. Center channels usually have a wider horizontal dispersion pattern but limited vertical dispersion, so you'll have to point it down at the couch. A dipole is going to leave you with a gaping coverage hole and very funky sound. The only thing I can think of that will otherwise work would be a Klipsch WDST surround (which is essentially a very wide dispersion monopole) but the tonal balance and efficiency will be radically different from your fronts unless you EQ the heck out of things. Either that or a pair of in-ceiling speakers.