Pipeline,
Awesome sketches and explanation!

"RealTrack section data in the SGU" - The "Position" data of RTs segments... sounds good, but
"can play those sections direct from disk"

Here is where I am a little bit concerned. External USB drives, especially spinners.
Because of various USB chip mfgs there might be a huge inconsistency with Random Access, because each minor change in arrangement will require a re-generation which is randomized in BIAB itself, therefore "seeking" those "different" sections and restructuring "what needs to be played" might take longer? I am just guessing here.

A gut feeling tells me, those who have BIAB on fast internal SSD drives or external SSDs with Thunderbolt 3 or higher, will actually see things speed up and process would be cleaner significantly.

I am very curious and a bit nervous how PG team will implement Utility Track functions. This was and still is my highest priority "wish" smile I see how mechanism you described will be the way to go for something of 10-15 tracks or even more in BIAB (legacy + Utility) and bar to bar regen.

P.S. Pipeline, are section transitions smooth when you do this in Reaper? Zero crossing and all that good stuff?