In the process of doing this, so I'll answer my own question. No need to drain the coolant to do this. You actually lose very little coolant, maybe 1/2 a cup's worth. I did plug the pipe up with saran wrap and rubber bands to temporarily seal. I didn't leave the metal pipe open to continue dripping.
Did you disconnect the radiator or just lift it out of the way? I haven't gone in there yet...
I have it! (5/6/2006) 2005 CO, LSS, Touring, hard top.