Sure:
Program a key to be your blackout on MyDMX and it's resolved. You can hit it whenever you want. "ON THE FLY!" Whee!
Seriously, one of the issues with a band playing to something else is as you stated: bands are not necessarily going to be tempo-perfect, so they are going to speed up and slow down. it's just a fact of life. The best way to try to get around this is the following ideas. These vary depending on what people feel comfortable with:
Click track. Typically delivered to the drummer. Drummer locks to tempo. Drummer will often have click track delivered to a set of cans to be worn for that song(or songs). Click track can be delivered via multiple methods, including CD, DAT, MIDI sequencer or even a pre-sequenced track(sans click track). Click tracks are rarely delivered through wedges. This overall provides the best methods of syncronization. Also, putting a pre-recorded track INTO their monitors helps as well. This is what will generally keep things in sync, ASSUMING you're using pre-recorded tracks of some sort.
What you could do is the following:
MIDI sequencer, with a rough but fully working version of the song on it. Say, Sonar or something else inexpensive and affordable. MyDMX is on another machine. Sonar generates the click track. Following me?
TWO LAPTOPS: One sequencer, one MyDMX. 2 MIDI interfaces.....
Following?
1 PRO Audio interface is recommended. USB or Firewire is fine, you don't need much especially if you're mainly using this Sonar computer for triggering MyDMX and not for backing tracks.
The MIDI sequence is for song programming reference. Hell, it dones't even need to be MIDI, it could just be a raw but accurate mix so you can work out timings. Then you can program MIDI triggers into MyDMX and then have Sonar trigger them. SEnd click track to appropriate monitor sends. In my case, I'd burn a matrix input and send that to the drummer to save an input on the console.
If the band can give you a few seconds per song to load the right song, you're good. Hopefully they do set lists, but I get what you're saying as many bands simply do not do this, they just "go fer it!".
Syncing to something else restricts bands and perhaps the natural energy they may be trying to build off of. This is part of the live act.
My other suggestion is to get a dedicated lightng director/programmer/operator to deal with this. This is your best overall option. That guy can pay attention to what's going on and if necessary, advance the scene as needed even if the previous one was not done yet.
Another option is to encourage set lists and stick to them. This is what the big boys do, but it's not always a "set in stone" thing as many times they'll switch it up on the fly anyways so you still have to be ready.
I think my stupid-slow blackout is programmed to the "-" key. Easily accessible, quick to reach but not so accessible as to cause accidents.
You're not in a win-win situation. You'll have to come up with compromises to make it work. It CAN be done. But, if you're doing TONS of scene changes, you're doing too much. There might be an intro scene, outro scene, verse scene(s), bridge scene and chorus scene(s) and if applicable, modulation scene. Just have a "key strokes" guide per song and you should be OK, then end each one with a blackout, then it fades to maybe a half-on all white or something so the audience can see them again.