So - It appears that the way forward would be to:

1. Prepare the mixes of each song (with Bass & Drums removed) and save them as MP3s (or some other agreeable non-midi format).

2. Trigger the MP3s through the Yamaha Multi-12, with the click generated via midi (and sent to headphones).

The next question then, is this: If I design MP3s to be triggered in this manner, can they be slightly stretched or compacted without varying the pitch?

Any thoughts?

Thanks Everyone!