Your fantasy to see the transcription of the stem bass line is possible. Yes, there is a feature in BB where the user can write in the actual notes of the bassline. Or better for folks like me that don't write notation, import the transcription midi data. Either method is possible.

The process has improved in 2025 but it's been possible as far back as 2014. With the introduction of the Stem Splitter and combining it with the Playable RealTrack feature, BIAB can utilize Playable RealTrack samples on the same track as live recorded audio or separate stems tracks. Stems from the Stem Splitter are recognized by BIAB as audio and function within the program as an Artist Performance File.

This feature makes it possible for you to enter notation and view it in BIAB for any live recorded audio.
This feature makes it possible for you to attach a Playable RealTrack sample, or VSTi to live recorded audio

Even though there are still some RealTracks that haven't had exact matching instrument samples recorded yet, all RealTracks have an associated Playable RealTrack assigned and work with the Playable RealTrack Feature by allowing underlying midi data to be on the same track with RealTracks or Artist Performance Files and Artist Performance Tracks.

Stems tracks, Artist Performance Files, UserTracks and RealTracks can all have Transcription data allowing views for Notation, Piano Roll, the Big Piano, the Guitar/Bass/Ukulele Fret Board and other views. The fact that Stems Tracks are recognized by BIAB as Artist Performance Files makes this possible. I've attached PG Music's description of the Artist Performance File UserTrack posted back in December 2013.

Attached Files (Click to download or enlarge) (Only available when you are logged in)
Artist Performance Files.jpg (27.14 KB, 89 downloads)
BIAB 2025:RB 2025, Latest builds: Dell Optiplex 7040 Desktop; Windows-10-64 bit, Intel Core i7-6700 3.4GHz CPU and 16 GB Ram Memory.