I’m actually trying out your patch now. A couple strange errors and observations:

>>> f"{'{'}"  # Why is this allowed in an outer format expression--
'{'
>>> f"{3:{'{'}>10}"  # --but not inside a format specifier?
SyntaxError: nesting of '{' in format specifier is not allowed
>>> opening = "{"; f"{3:{opening}>10}"  # Workaround
'{{{{{{{{{3'
>>> f"{3:{'}'}<10}"  # Error message is very strange!
SyntaxError: missing '}' in format string expression
>>> f"{\x00}"  # It seems this is treated as a null terminator
  File "<fstring>", line 1
    (
    ^
SyntaxError: unexpected EOF while parsing
>>> f"{'s'!\x00:.<10}"  # Default conversion is the null character?
's.........'