Using the surround back channels for zone B?

I just picked up a B&K AVR 307 here on AudiogoN. I presently have a B&K AVR 305, so I'm pretty familiar with this unit.

I'd like to use the 307 in a 5.1 setup and utilize the back channels as a power amp for Zone B (speakers in the next room - music only). The idea would be to take the Zone B pre outs and feed them into the back surround channel direct inputs. Has anyone tried this? I've heard you can send the unit to B&K and have it wired just for this purpose, but I'd rather not ship it back and forth across the country. Is there a simple "under the hood" mod for this where I move/remove/add some jumpers?

I previously had a 507 S2. You do need to send the unit back to be able to amplifier your zone B speakers from this unit. I had the same issue as you about not wanting to incure the cost and time to send it back. What I did was purchase a second 2-channel amplifier for the second zone and ran the pre-outs to the second amp. The disadvantage to this is that you can turn the second zone speakers on and off (by just turning the second amp on and off), but you are subject to the volume level in the second zone being the same as in the main zone. There may be another way, but if so I was not able to figure it out (after several calls to B&K, who at least are very nice and curtious).
Adjustable volume control - Zone 2: here's how I did it with a fixed Zone 2 OUT.

I connected from my Proceed AVP2 FIXED ZONE 2 OUTPUT to a Yamaha recievers L/R MAINS INPUT. My Yamaha's MAINS/Pre-outs went directly to the 12-channel amp.

That added the adjustable volume controlled through the Yamaha's pre-amp. The sound is much better throughout all 6 zones. Before, the Zone 2 OUTPUT signals from both the Proceed and the Yamaha was slightly too low for the 12-channel AMP's input sensitivity. All ZONE 2 speakers throughout the house sounded washed out.

With the additional gain provided by the Yamaha pre-amp into the 12-channel Zone 2 AMP, the difference made all the time and money installing Zone 2 worth the efforts. Each zone has separate wall volume controllers. Works great.

Using surround OUTs may work for you in a similiar way.