This is how I do it using BIAB 13.
Go to Edit, select Unfold (convert to 1big chorus)
As an example if your song was 24 measures played 3 times, it will
Now be one song 72 measures in length.

Next highlight the first measure and press the F5 key. This will
bring up the Edit Settings .....box. In the middle of the settings page
select Bass and highlight Mute. Click OK

Select the bar that you want the bass to enter, highlight it, select
Bass and select Back to Normal. Click OK

That should do it.

As far Keeping the bass from changing, go to the Freeze icon,
either Freeze all tracks or just the bass track.

Hope this helps