@nprofile1q... Just wanted to let you know that I got things working.
The only thing about Dear ImGUI itself that confused me was that tex->Pixels always points to the _full_ image, not just pixels for the current subrect, and therefore the backend is expected to manually extract the relevant pixel data itself. I have no idea why I assumed that in the first place, but I only figured it out during my nth read of the Vulkan backend diff when I finally spotted the memcpy code.