Interestingly this is why I gave -1 in SE-0386 review.
In the new implementation, I tried to not modify the compiler itself. Providing parsing through SwiftSyntax is mostly for developer tools (like IDE) to provide diagnostics, which is one of the remaining pieces of the original PoC.
I believe there’re other ways to achieve this functionality, eg. we may have a new macro type that’s effectively no-op but to provide extra information in a designated format. Or like @Karl suggests we can supply modules through external providers to the compiler as a plug-in (I had considered similar approach, but gave up mostly because it’s challenging to design a module-based global cache system).
Implementing these, however, is beyond my ability as an individual developer. I sincerely welcome more thoughts on how we can do this non-invasively, and support on related infrastructure.