Here is what I would love to do.
 
1. Select a passage filled with tempo changes, perhaps as a result of fit to improvisation. Loads of rubato in it, in other words.
 
2. Run a function that:
 a. looks at all the tempo changes
 b. grabs either the median or average tempo (user's choice)
 c. has a slider that you can set between 0 and 200%, where it would push the value of all the tempo some percentage of the difference from that median/average tempo.
 
In this way, you could either increase or decrease the rubato effect in a passage.
 
(Aside: it's annoying that CAL script can't do tempo....)