Time stretch on a curve? Solved - Sort of...

If I think I'm following you correctly, you need to first set up a tempo map at the tempo change/rate that you want and then fit the audio to that tempo change/rate.
 
You can set up the tempo changes using Project-->Insert Series of Tempos. Once that's done open Audiosnap on the relevant audio and use "Clip follows project" to adjust the audio.
 
Remember that the bounced render will sound much better than the online render.

The tempo map process looked the most promising, as I could map the tempo from a faster to the original tempo in increments making a curve.  I would do that after mixing down to stereo though, so I would not have to synchronize this change across 48 tracks.  How I actually did it was a bit more brute force, however...  The intro from the previous night was at the right tempo so I opened both projects and cut the intro (48 tracks) from the first night and pasted it into the second nights performance, nudged it just right, and voila (or as they say in Texas vo eela), a undetectable edit.  I love Sonar. 
 
Also, of interest.  A female member of a vocal trio was supposed to sing "This is the song the wise men sing", but sang the lyrics for the second chorus "this is the song the carolers sing", catching herself, and singing "ca-ise men sing".  An obvious goof.  I took the correct word from one of the male singers, pasted it in place, melodyned it to the right pitch, modified the formant slightly to make it sound a bit more girly, and again, a perfect fix that is undetectable in the mix!  Go Sonar.