Sorry, I’m late to the game here, I didn’t realise the significance and review this PEP until this bug landed in front of me.
Debian (and all derivatives) are going to have to use the technical limitations opt-out from the standard installation path.
We support a multi-arch install layout where the same package from multiple architectures can be co-installed. This works by putting files containing architecture-specific content in paths that don’t collide (either architecture-specific path hierarchies or architectures embedded in the filename).
The logical thing to do here would be to allow embedding the multiarch tuple in the build-details filename, e.g. build-details-x86_64-linux-gnu.json. It’s a little late to make that change upstream for 3.14, we’ll have to just deviate for now and start pursuing this change upstream for 3.15/3.16.