Like you said, I'd be leery of the risk to your song catalog more than anything. On Dale's point, I always try to remember that the DJ is not the star here. Look at it from the singer's point of view. I feel my primary job is to make them sound as good as I possibly can. The "show" is secondary to that.
But, what your DJ is proposing is technically possible. The problem you will run in to is when you change the BPM, you will have sync problems with the lyrics. Try a couple of songs and see what I mean. MTU has always been the best at keeping sync with the lyrics and tempo change has been a bear for them.
I just can't imagine trying to make that fly with a crowd. It would go over like a turd in a punchbowl here
