In general, when you weld aluminum, you either heat treat it after you weld it or use a non heat treatable Al. 6061 is a heat treatable alloy. Assuming It starts at heat treat 6 (T6) welding 6061 T6 changes it's yield strength from 95,000 Lb/in2 to something between 95,000 lb/in2 and 30,000 lb/in2 around the wield. Probably a lot closer to the bottom of the range than the top.

When people say welding heat treated aluminum is not a good idea, we are mostly reading it right out of engineering or aircraft repair handbooks.

Assuming the fact that I don't know a thing about the loads placed on that part of the mast, would it be of your opinion that this is a futile endevour?

Could you send in a photo of the damage, or at least describe it's size and location on the mast? I saw someone sailing with a large dent near the base of his mast. When I asked about it, he said that it had been there when he bought the boat 10 years before, and hadn't caused any problems. Maybe welding it would make it weaker more than help?? A photo or description would help some of the experts on the list answer your question in a more informed way.
Is your trip to Key Largo still happening?
Jack Hoying
Fort Loramie, Ohio
Prindle 18 #1645
http://www.bright.net/~jmhoying/sailing/prindle.htm

"Assuming the fact that I don't know a thing about the loads placed on that part of the mast, would it be of your opinion that this is a futile endevour?"
This reminds me of a photo that I saw once. I think it was of Dan Berger using an old Hobie 16 mast to remove some concrete sidewalks on his property. It was like having a 26' prybar. Imagine the leverage you can get with a couple guys hanging on one end. As I recall, the mast was not harmed.
Jack

I'll work on the picture... Don't have a digi cam down here in Raleigh, but maybe a friend will let me barrow it for the day.