Here's what I ended up doing...
(2) Behringer C2 mics are mounted above the chamber to pick up all the air-mixed speaker output from the organ.
These run to a Behringer ULTRAGAIN PRO MIC2200 mic pre amp
Mic pre-amp -> a Lexicon MX400, which is set on 'stereo in - 4 channel surround out
A pair of Behringer A500 amps drive a total of 8 Infinity 6X9 co-axial (car audio) speakers
mounted in standard 6X9 MDF boxes. These are wired in series-pairs for a total of 4 channels - (2) front and (2) rear. The speakers are placed all around the room, with the rear speakers at the front of the church (farthest away from the [gallery] organ).
I've been experimenting with settings on the Lexicon. Adding an excessive amount of decay sounds odd. I have had best results starting with a Surround setting called 'Big Ambience' - then matching the decay time to the natural decay time of my room which is 1.25 seconds. This setting has a short decay, but relatively high diffusion.
Aside from reverb, this affect helps with another problem. Non-wind instruments lack "presence" when the point source of the speakers is at one end of the room. Where-as with a real pipe organ, the sound envelopes the room. This system seems to compensate some.