Don't bother reducing workout volume at all. Climbing is all about sustained effort--as you know--and for somebody adapted to sustained work it is very hard to overtrain. In other words, you are already partially conditioned, so don't worry about overtraining. Although it is possible to do it, it is very hard. I will give you a couple examples.
1st example is a 51 year old client of mine. He is a doctor, and he had been doing Thibaudeau's Star Complex workouts with me (only 30 seconds rest), as well as doing 4x weekly 30 minute non-stop plyometric conditioning workouts ("Insanity", yes the infomercial workouts--this was because his wife did them and wanted him to keep her company--happy wife, happy life. He did not really enjoy them). He was putting up 315 lb deadlift, 385 lb rack pulls, 265 lb half squat, and 200 lb front squats in the Complex workouts at a body weight of 150 lbs.
He did this for 3 months before hitting the wall in overtraining. We backed him off, and he's just fine now and doing great.
I had another cyclist client who was 63. He was doing high volume bodybuilding workouts, deadlifting 225 x 6-8 reps, and squatting 225 x 2-3 reps as well as doing high volume cycling work. He was just fine. Mobility issues we were working on, but other than that nothing.
2nd example--female figure client. She is currently eating less than 1400 calories a day on training days, is 3 weeks out of a show, extremely low bodyfat, and is training 12 times a week with 6 weekly conditioning sessions, 6 gym workouts, and 6 days of slow steady state fasted cardio in the morning. Her metabolism is raging right now and she is feeling fine, albeit very hungry :). Her weights have not dropped seriously yet. She is in no way overtraining, and we are keeping tabs on her daily.
Now, I'd like to add a disclaimer about her--I only picked her up as a client a week ago when she had a falling out with her old coach who dropped her and was previously giving some very dangerous and terrible advice to her that she refused to follow, which lead him to drop her. She was behind in her prep by a significant amount of time due to this coach, and therefore the work that she is doing is not my preferred method of preparing OR my preferred diet plan. I have had no control over her training up until she asked me to coach her. This is a harsh "catch-up" measure, and is only being employed because it is so close to show time and she is behind. That being said, she's feeling fine and her weights are flying. No overtraining. We will only use a couple more weeks of this, of course.
Lesson: overtraining is very difficult to accomplish in people who are adapted to sustained effort style volume. Of course they worked up to it over time, but there is no reason that you will magically overtrain. If these people can thrive on workloads like I described, then you should not worry about a little complex work :). Just make sure you've got solid form on the complexes and exercises and pay attention to mobility and soft tissue quality work. Those are usually the things that start to bite you in the ass soonest.