I second (or third ;-) ) that request.

Just as in Sonar (actually dating way back to pre-Sonar Cakewalk days), one can "insert a series of tempos"---by specifying starting and ending time, and starting and ending tempo, and the frequency of change (every beat, every half beat, etc.), and get a ritardando.

We should have that in BIAB too, and it should work for RTs and RDs as well.