It's not canon, but the TNG Tech Manual
has implied that impulse engines do generate a subspace field (to reduce the mass of a vessel), and the saucer can coast at warp for quite awhile after separation before gradually dropping back to sublight.
But ideally, I think saucer separation is really supposed to be performed when the ship isn't moving at all
and before combat actually begins. The saucer would then be nowhere near the battle theatre and well out of harm's way--under ideal circumstances.