Is it possible for Logic to record a wet track made in Mainstage?

I have no idea if there's built-in solution but I bet that the free SoundFlower subsystem would let you do this.

See http://cycling74.com/products/soundflower/

Why bother? Logic is designed for recording music. MainStage is designed for live performance. Different tools for different jobs. 🙂

I am always dismayed when I see the ubiquitous "why bother" response, which I see quite often on the software development forums as well.

If people only ever used tools for their originally intended use, there would be very little progress in the world.

I can easily imagine how, after configuring a patch and just playing around, one might come up with a musically interesting riff or an interesting sound effect that one might want to immediately capture for later use or experimentation. Indeed, BECAUSE MainStage is designed for live performance, it lets you do some things quite easily that are much more awkward to setup in Logic without a lot of effort and preparation (for example, splits and layering and using multiple keyboards simultaneously)

It happens to me quite often when I'm practicing on the grand piano, which was also not designed for recording what I'm playing. Fortunately, Don Buchla invented the PianoBar which lets me easily capture what I'm playing into a MIDI recorder.

Similarly, SoundFlower lets people use tools that were not originally designed to work together in the audio world on a Mac. Do you think the PianoBar or SoundFlower should not have been invented?

Sigh

Not sure if this would work for you, but MainStage is set up to record its own performance. You could bring that file into Logic.

The suggested use of Soundflower is a good one as well, if you want to record directly into Logic.