Something like a donation dashboard might solve your problem? They could let the donor set an optional maximum monthly contribution. Of that maximum contribution, the donor could set how much of it goes to each tip. The giving page seems like a good place to put such a thing.

As for tipping a charitable organisation that might not be on gittip, perhaps setting up proxy accounts where the monthly tip is donated by gittip would work. Of course, this would be susceptible to a social engineering attack where a scammer could fraudulently claim to represent the organisation and ask for control of the account.