Give it a shot I'd say - there's not much to loose in a sense and you can easily readjust things around a bit.

So you're recording from both the mics at the back, mixed with the mixer?  I'm not sure, but it sounds like you'll be recording the guitars twice, since you're recording from the mics that go into the mixer, then from the output of the speaker that goes into any nearby mics.
Also am unsure of the effect of this type of feedback.
Note, I know zilch about this stuff.