What if we just allowed you to specify the test path as a diagnostic (like we do in add_reserved_diagnostics, but delete after).

Descriptor already understands mapping between test case names and test paths. Could add_histograms create Descriptors and call ToTestPaths?
In the distant future, Descriptor should evaporate, but that will be easier the less we rely on uploaders to compute test paths. If add_histograms uses Descriptor, then we can unilaterally migrate test paths and remove hacks from Descriptor without requiring uploaders to change anything.