In reply to krank:
I am definitely not a training guru, but I would have thought that it's not so much a question of avoiding putting on any muscle mass, as putting on the right muscle mass.
Even though muscle increases in weight faster than it does in strength, I would have thought that most people could stand to put on muscle mass in their forearms and shoulders (to a lesser extent) and still improve their strength to weight ratio, as the weight in your forearms and shoulders is low compared with the weight in, say, your legs, that you need to carry up the climb with you.
Just to make some numbers up - let's say your forearm muscle mass is 4kg out of a total body mass of 70kg. Let's say you improve your max load by 10% but put on 50% additional weight - that's increasing your body mass from 70kg to 72kg or 3% overall. So that's a 10% gain in strength for only a 3% gain in weight overall.
I don't know what actual numbers are, I made those up. But make sense in theory?