I do it all the time, on a circa 2008 mbp, seems to work just fine.
you'll want this:
http://cycling74.com/products/soundflower/
the way I do it is that I go into audio midi setup and I create an aggregate device joining my audio interface and the virtual interfaces of soundflower.
Then in mainstage, I'll create two busses: one with an output to my audio interface, and one with an output to soundflower, and I'll send my channel strips to both busses.
Then in logic, I'll load up the same aggregate device. Then I can record multi-tracks from my audio interface, and I can also have a separate track that gets the outputs from soundflower ... so I basically get mainstage on one track and the rest of the band on their tracks. Be sure to mute the main outs in Logic.
works like a charm. runs the cpu pretty hard. you'll want your power adapter for sure.