Hello srw647,
On "Organ settings | Rank voicing and surround/3D panning" screen, select the "Perspective mix: source persp to output persp 3 (rear 1/main)" adjustment, and turn it up to (e.g.) 100% for the desired ranks. If you wish, you could also truncate the reverb for that perspective using the "Output persp 3 (rear 1/main): release tail truncation length" adjustment.
Now open the "Organ settings | Rank routing ..." screen, and in the right-hand pane the "Rank output perspective 3 (rear 1/main): Destination ..." setting specifies which mixer bus group that perspective (of the selected rank[s]) will be routed to. By defaulted, it's routed to a group which ends up going to the "Master mix 7: Perspectv mix: outp persp 3 (rear 1/main)" bus on the General settings | Audio mixer" screen. Hence if you leave it routed there (as it is by default), then all you would need to do is to select the desired reverb for that particular mixer bus on the mixer screen.