I know its a PITA to get to the rears. You could tap the fronts with nearly the same results tho, just a longer signal cable run. Fronts are easier to get to, but the cables will be harder to run.
It really is imperative to get inputs from both the left and right tho. Its just wrong to get all yer bass from just one channel. Even tho bass is largely non-directional, there are bass frequencies coming from both left and right channels that must be fed to the sub. Not only that your sub output would suffer by 50%, your front speakers will be imbalanced, interior phase of of the bass will be off, etc... its just wrong to try to feed it from one speaker level input. Don't even think about it.