Interesting thread ... surprising diversity of responses.

My 2c is, by the 2nd half of a charity-type ride you have a good idea of the capabilities of the riders around you, that is, they are probably similar to your capabilities. Every such ride I have been in naturally forms into pacelines (at least near the front, that is).

Personally if I catch someone who's been dropped off a faster line, I shout hi and grab his wheel without hesitation, and expect to take turns pulling until one of us gets dropped or we collect more riders and build our own line. To me, that's part of the whole group spirit of such a ride.