This sounds really useful, and to me conceptually similar to PEP 712: Adding a “converter” parameter to dataclasses.field (though not the same since converter applies to all values).
It’s like the difference between dataclass.field’s default_factory and converter: One sets a default value from a 0-argument callable, and the other uses converts the initial value using a single-argument callable.
So for naming, I think converterdict or converter_defaultdict may fit.