p.s. the procedure I ended up using was in fact this one; it was simpler (not requiring the Go language) because the 2 images required are both directly available from the payload file… that is, the 2nd half of the procedure, with mounting all the individual images, was also not required.
I’ll be posting more about the overall experience of getting T (dev) to work on the guacamole (edit: here). Maybe most important: although dtbo.img was the same (checksum) between the latest dev S and T versions, the vbmeta.img was different:
From reading through all the OnePlus “touch screen doesn’t work” postings today, I would guess a lot of those problems could come from flashing the wrong version of the vbmeta image (or not flashing these 2 images at all).