OK, I have looked closely at this. It appears what we are seeing is a way to permit the midi tracks in the BIAB VST within Reaper to use whatever sound-source is available rather than the Win10 GM player. To do this, the BIAB VST is first inserted on one of the Reaper tracks. Apparently, the midi tracks on the first page of the BIAB VST output to the standard BIAB midi default channels:

Track Channel
Bass 2
Piano 3
Melody 4
Thru 5
Guitar 6
Strings 7
Soloist 8
Drums 10

Next, it appears in Pipeline's clip that we just have to use Reaper routing to send the individual default midi channel from the Reaper track which has the BIAB VST on it, to a separate Reaper Track, which has a sound synth on it which then receives the midi channel. Could it really be that simple! Oh, then we have to mute the original BIAB VST tracks (I think).


BIAB – 2025, Reaper (current), i7-12700F Processor, 32GB DDR4-3200MHz RAM, 1TB WD Black NVMe SSD, 2TB WDC Blue SSD, 1TB WD Blue, 2 TB SK NVMe, 6 TB External, Motu Audio Express 6x6