John-Luke, I just noticed exactly the same thing.

Today I was programming an accompaniment for Harry Chapin's "Cat's in the Cradle" which has 2-beat bars in fill positions leading into every single chorus. The song has really nice drum fills at these points providing a perfect crescendo into the choruses. No drum fills are possible in BIAB in this scenario, therefore unfortunately there can be no drum fills to lead into choruses of this song.

I wish there were a way because the build and drum fills are a key element of this song (and many other songs, such as "Your Song" as you mentioned). The RealDrums tracks are capable of providing very pretty fills which would reproduce these moments in the song beautifully if only this were possible.