We all know how simple it is to have a band follow a sequencer.  Provide a click track and yada yada yada.
 
But, is there a practical way to achieve the opposite:  have the sequencer follow the band?
 
Can Sonar (or any DAW) be set up in such a way that it can be triggered by, say, the drummer, and then follow along with the tempo set by the band.  My intuition suggests that there can be a way to achieve this via the transients, but I can't think this through to the end.
 
Why, you ask?  If a sequencer could follow the band, the DAW could be used to automatically trigger all sorts of things that humans have to do now:  synth patch changes, guitar effects, vocal effects, instrument levels, and most intriguing, DMX lighting commands.
 
Any ideas?

I don't think so. In midi land, something has to be a master, and something a slave. You need to be able to generate MTC on the fly to have what you are suggesting, and while that can certainly be achieved by most midi devices, it is a predefined thing, not capable of locking in on a groove or tempo fluctuations. those things have to be programmed in beforehand and hence,  the midi devices are not "listening" at all to what is going on, just chugging away as instructed. Th band is going to have to be the slave and follow the sequencer. As long as midi and the band are on the same page, you can program events to happen at a predetermined time, but MTC is the timing master, not the drummer.

I haven't used any, but there are tools for that.
 
Here's one that popped up.
http://www.circular-logic.com/products.html
 
Google is your friend ...
 

I should have done some investigation before posting.
 
It took me about 10 minutes to find the solution.  A program called InTime by Circular Logic will do exactly as I described in the OP.
 
It is free to download.  I am going to give it a try.

Google is indeed our friend.  It took me a few tries to form a query that got me the results I was looking for.  Thanks for the response!

Ableton has a Max4Live plugin that can do this too. I saw a great vid of the sequencer chasing the inputs from a drummer's kit and doing tempo manipulation on the fly. 

But that circular logic program requires DirectX. Has anyone used it on Win10 yet? It has DirectX v12 which may have some compatibility issues.

Glad you found something.Didn't know that one was out there.

I would be very interested to hear how well these solutions work in the real world. I have found any kind of advanced external sync using multiple tempo changes problematic, let alone on the fly with an essentially infinitely variable master clock!