Rich:
Very Possible. I did that 4 years ago and the radar has been valuable on occasion even on the Great Lakes.  I used an extension ladder and a halyard as a safety line. Worked well for me - although I made way to many trips up/down the ladder for stuff. A work bag with the required supplies and tools would have been a good idea for me.  If someone is on the boat you need to get them to stay in one place. Gets kind of wiggly up there with someone moving around the boat. I tied the top of the ladder to the spreaders to keep it in one place too!  Getting the wiring down the mast was a little tricky.
I think if I had to do it again I would have dropped the mast to get it installed and done some other mast work while it was down as well.
Good Luck
 

I've never done it but I do know you can't just drop the cable down the mast or it will bang around in the mast when at anchor; has to be anchored in some way.  Had a friend with this problem.... very annoying clanking noise when they tried to sleep. Plus, you say the mast is up but you don't say if your on the hard or in the water. Generally considered a very bad idea to climb the mast when on the hard.