Band in box treats slash chords like this:
Bb/C (C7sus 4, in fact): C is the root, 5th is the third of the Bb major triad, in this case it plays C and D which is C2 and not C7sus. So, alas, no solution here.

If you want a real Bb13b9 sound from your band you could write G/Bb. The bass player already knows you’re emphasizing b9 and natural 13. Being the sophisticated person he is, the bass player will definitely also play the dom7th, which is Ab! The most used slash chords are known.
But in Biab, the bass will mostly play the notes Bb, D and B (or Cb). This sounds weak, because you miss the bass playing the 5th or the 7th.

I believe we -as users- are always looking to find ways to improve the performance of biab. In fact, it should be the programmers, pgmusic, who should make it easy for us. We shouldn’t be the ones to try and make things work. You and me have paid our share.

Biab, Kontakt, Sampletank and lots of nice libraries, from Fluffy audio to Abbey Road drums.
Check out these great contemporary Jazz Styles: www.jazzstylezz.com