I think it would add some significant flexibility to Python to let users
define custom string prefixes. What I mean by a string prefix is,
a letter prefixing the string literal, modifying the behavior of it, [...]
The feature isn't present. But it isn't really clear what you want either. The current string prefixes are not dynamically analyzed. Do you want static hooks for the lexer? A string method to register new prefixes? Do you want to specify new string prefixes in the existing framework?
I think string prefixes aren't something that we want more of, this is already complicated enough: