Changing callouts is the same as changing songs. Find an audio clip that is approximately the same length and replace the offending callout. I agree with your "We're gonna do a little number for you now" opinion - I cringe a little every time I hear it.
For anyone interested in trying something new on their Pro, I've got a modded version of 1.41 that has live versions of all the songs. I've replaced 3 songs (Hotter Than Hell. Calling Dr Love, Love It Loud) with other tracks (Makin' Love, God of Thunder, Strutter respectively) but the other 7 titles remain the same. All the songs are original live recordings and sound WAY better than the re-recorded crap that the game originally came with.
One note - when I originally replaced the songs in v1.28, Stern had not yet implemented the "You Wanted The Best and You Got the Best" quote on game start. One of my tracks (Detroit Rock City) has that quote in the beginning so you may hear that quote twice if you select DRC on game start.
Anyone interested can PM me for a link.