>>Maybe a rear channel with a little delay added (reminiscent of the old quadraphonic "ambient" speaker idea).<<
many many years ago, I saw a diagram in a late 50s hifi book where the author suggested a way to broaden the stereo sweet-spot & pull the stereo image forward of the flat plane between the speakers. what he did was to add a third, fairly crappy speaker, behind the listener. it was connected between the two "reds", i.e. the left & right +ve terminals of his amplifier. I experimented with this a lot, trying to find the right size & impedance of speaker so that this passive arrangement would be balanced in terms of frequency response & output level. currently I have a side-pod speaker from an old sony profeel video monitor, but anything of this sort of size will work with most home hifi setups.
we printed a schematic for this inside an album sleeve about ten years ago, although brian eno beat us to it with one of his ambient efforts ("on land"?)
anyway. for an audience-type scenario, the same thing would probably work provided that the third speaker wasn't too close to anyone in particular. I would "sky" it if possible- sling it from the ceiling at the rear of the venue. I would arrange for the "difference" signal to be derived properly, with either an isolating transformer or using both inputs of an op-amp to buffer the left & right channels into a mono power amp driving this speaker. then play some stuff through the system & wander around the venue checking the balance.
I've been meaning to do this for a while myself, but the last few gigs we've done in the UK have been in domed planetaria, with their own way of creating immersive sound.. :-)