Does this seem worth coding up in its current form?
No. The pg_upgrade utility is awesome and I have commended Bruce on multiple occasions about his work with it. That being said, the "solution" is to support in-place upgrades and our work should be toward that.
Yeah. Streaming upgrade is useful, but IMO it's a workaround for upgrade issues more than a solution in its self. Useful for people who want a conservative upgrade, but not that big a win. Sure you'd like to be able to downgrade again if it doesn't go right, but that requires a two-way sync, which introduces its own problems and failure modes.
Support for reading prior version catalogs in-place is what I see as the user-friendly end goal. Just start Pg12 on top of a pg11 datadir with the --allow-upgrade flag and you're done.
But I don't think I'm any keener to do the drudgery required to implement it than anyone else is... and I share others' concerns about the maintenance burden imposed, impact on future catalog change freedom, etc.