(note: I am not a member of the release team, just a dd who spots a load of build-depency issues in testing as part of running a derivative)
I thought testing with dose-builddebcheck already happened routinely but can't find it. Any historical perspective on that?
There is https://qa.debian.org/dose/debcheck/src_testing_main/index.html Unfortunately it doesn't distinguish between packages that have never built on an architecture and packages which have binaries on an architecture but can't be built on that architecture in current testing. A bigger Issue though is that failure to satisfy build-dependencies in testing but not unstable is rarely the "fault" of the package that is failing to build, but is the fault of build-dependencies that for some reason are not migrating. If the migration of the build-depencies is something that is "in hand" already then filing bugs against the victims is just a waste of everyone's time. On the other hand if none of the maintainers of the dependencies are paying attention then the maintainer of the victims may well want to be informed.