Thanks. I looked into Seedvault and gather one of its challenges is a reliance on API's that are deprecated and (in the meantime) becoming intentionally degraded. Eg. Apps can exclude themselves via android:allowBackup=false.
There have been calls to modify Seedvault to impersonate D2D transfer in order to bypass the restriction. Or I wonder if I can patch the OS to ignore/override that app manifest flag? (I gather some people are patching their APK's).
I wrote a little more about this here:
https://news.ycombinator.com/item?id=37774254