I remember Data saying in the first episode that saucer separation was not safe at any speed > zero (relative to what, I dont know), much less at warp. This seems to contradicts that "warp sustainer coils" theory.
Maybe this is one of the times when TNG tangled itself in it's own technobabble.
Here's the dialog
Encounter at Farpoint wrote:
PICARD: Records search, Data. Results of detaching saucer section at high warp velocity.
DATA: Inadvisable at any warp speed, sir.
PICARD: Search theoretical.
DATA: It is possible, sir. But absolutely no margin for error.
I don't think that means necessarily that there are no sustainers (I have no dog in that fight, by the way), just that separation at warp is potentially unsafe. If sustainers are there, the intention is pretty clearly to use them only in case of emergency already threatening the ship.