What you're trying to do is not much different from what Todd Rundgren did years ago and what Sting did with a Synclavilier in the early 1980s. Let's face it, you'll probably be using a laptop. Any multitrack audio software should be adequate for your purposes. You'll create sub mixes a.k.a. stems that have already utilized your equalization & dynamics processing. You'll then be able to balance all of those items against your live performance. So it's really simpler than you're thinking it is. You can just use your mouse or you can use an outboard MIDI fader box that can integrate with your software. I've been trying to talk a friend into doing that for years but she feels that people would feel ripped off. She's wrong. You've got the right idea, especially where you can't squeeze a band into in certain places.

Squeeze him there. Maybe he'll pass another. (does anybody remember that quote?) We are all bozos on this bus and everything you know is wrong.
Ms. Remy Ann David

While you have been given a proper answer, I would say that an iPod is simpler. Bypass the MIDI and click, requiring other hardware to work. Just use an iPod with a back-up CD.

We have a duo, and have been using tracks for years. Been through all the methods you mentioned - MIDI, Ipod, Multitrack playback, etc.
Firstly, MIDI sucks, even if you have a good synth to run it through. It’s also a pain to run live. Laptops fail, crash, etc. Ipod works, but difficult to see and use on a dark stage. It might seem cool to have each instrument on a separate track, but in actual use onstage, it’s a real pain.
What I do now is bring the MIDI track into Sonar, use good synths, like Dimension Pro to replace the instruments with samples, record the result and save as a WAV file. This is burned to CD, Ipod and Laptop, and used in that order. Triple backup.

It may seem like a lot of trouble to get a stereo backing track, but the result is head and shoulders above MIDI.