Just answered my own question - I didn't read the feature explanation properly. You have to select 'include acid information' when you render wave from BIAB.

Yes I have always preset my tempo stuff in Reaper before importing BIAB wave files. This new feature should make it easier though. Here's the exact wording for the new feature:

Added: Extra information written to audio files so that DAWS like SONAR, Presonus, Reaper etc. will read the tempo of the WAV file and stretch it if necessary. Note that "Include Acid info" must be selected in the Wave Render settings dialog (Press WAV button for this).