Yes, moving f-strings to the grammar would be a huge change, and not just for CPython.

I discussed it at the last PyCon with the authors of various editors (for syntax highlighting) and other tools that parse python code. No one was hugely opposed to it, and I think even one person was very excited about it. But it's not a step to be taken lightly. I don't think I've discussed it with any other Python implementors outside of CPython.

My biggest concern is that it makes naive string recognition fail. For example, this would become a valid f-string:
f'{fn('some string')}'
while
'{fn('some string')}'
is not a valid string.

I now I think I've completely derailed this bug report. I think the action item here is for me to finish up my better error reporting.