You're framing this as a question of "how much will this hurt my climbing"? I wonder if you could use this down time from actual climbing as a chance to focus more on pure climbing specific strength training, and possibly come out of the whole process a stronger climber.
Something I tend to do is all but quit route climbing in the winter, and take it up again in the Spring. In that "down time", I'll still boulder once a week, and incorporate some climbing specific strength training. I lose a lot of endurance, but gain some strength, and usually come back in the Spring as a slightly stronger climber. If you keep your strength up, endurance will come back in a month or so.
If you already have rock rings and a pullup bar, have you thought about doing the official rock ring training plan: http://www.metoliusclimbing.com/training_giude_rock_ring.html ? Metolius published a 10 minute workout plan that includes finger strength, pullups, lockoffs, leg raises, all climbing specific stuff. It all done from a hanging position, so it shouldn't tax your lower body, and it would build a lot of climbing specific strength. And since you're planning on not climbing as much anyway, you could try and hit it a couple times a still have several rest days (at least for your upper body). You might actually come back stronger after the marathon, and be able to train endurance on routes on top of a better base of strength.
I feel like something like this (10 minutes of pure upper body strength training a couple times a week on rock rings), plus one gym climbing session a week, would integrate with your marathon training. But you'll have to listen to your body as you go.
