Bear in mind that the mast is part of a suspension system that includes the standing rigging and the deck or keel upon which the mast is stepped. So the weight aloft is supported by this system, rather than the mast alone.
I won''t bother to do a thumbnail calculation. I''ll merely point out that America''s cup boats, which are engineered so that mast and rigging are "just" strong enough to bear the loads expected from maximum winds specified in the race conditions, with the stays and shrouds tensioned to near 100% of their working load capability, often have a person aloft to sight wind upcourse on the upwind legs. If these carbon fiber masted boats with rigging tightened to within an inch of its capability can withstand someone aloft beating to windward, I suspect that most cruising boats with aluminum masts and rigging capable of sustaining three times the static loads placed on it could easily handle someone aloft as well. The point other posters make about how well you might be able to hang on is, of course, a good one.