I'm much more interested in the typing community's feedback; runtime-wise I'm sure something will be doable. If this style is accepted I volunteer to write the runtime implementation.
A tangent, since we're discussing implementation: I'm the person who initially contributed slot support to attrs, back in 2016 (phew), after which it went through several iterations by myself and other people before being ported to dataclasses. It's not possible to convert an existing dict class into a slot class so the class needs to be cloned, like you mentioned. But in attrs, cloned classes work with bare super on all Python versions we support (including PyPy) since we maintain the gnarly workaround code for it. In any case, I don't know yet if the `__mro__` of a class can be changed at runtime or if the class needs to be cloned.