After reading about other people's use-cases, I' now:
-1
I think that all that's required for solving Guido's original use case is a new warning in pylint, pep8, or flake. PEP8 could be updated to discourage the use of automatic concatenation in those places.
The warning would apply only to automatic concatenations within parameter passing and structures, and not to assignments or formatting through %.
Doing it this way would solve the use case by declaring certain uses of automatic concatenation a "code smell", and automating detection of the bad uses, without any changes to the language.
All that Guido needs to do is change PEP8, and wait for the static analyzers to follow.
Cheers,