Don't know if this helps, but if you go to the "Count-in/Metronome" options screen, look for the field "Lead-in Type". There you can actually select to have the lead-in be either the A or B drum fill from the style you are using. You can also check the box to use a 1 bar count-in rather than 2 bars.
There does seem to be a problem with this when rendering to wave file, at least if you select a one bar lead in which is what I want.
If you select the one bar lead-in, 'b' sub-style fills, it plays perfectly from within BIAB, but when you render to a WAV file, instead of a one bar lead-in you get a two bar lead in.
Not a huge deal, I can trim the extra bar after the fact, but seems like a bug?