Have you tried using the Audio Chord Wizard? It has a tempo mapping feature that works like this:

1) you click on the beginning of the first bar to identify the starting point of the tempo

2) you tap out the beats as the song plays

3) when you quit ACW to send the new tempo back to BIAB/RB, you get several options for dealing with the dead space at the beginning.
(delete it, expand it to a full measure etc)

go to HELP and search for ACW or Audio Chord Wizard or Tempo Map