I would like to register a custom type in general, rather than with a specific keyword library.

I like creating keyword libraries for certain, more complex, data types. Keywords than include factory keywords, operator keywords and some for managing storage. Other keyword libraries can then refer to these types and take them as argument, in which case the converters are super handy, but they aren't available at that place.

In my current solution I reverted to subclassing TypeConverter but this is not part of the public API. It would be better to have the option to register a custom type in general, so that any keyword library that uses the type can also benefit from the converters.