ORM offers two things:

• lets you convert a property type into another one: this is the JPA @Converter support and is good for a lot of use cases. It is at the higher and object level so I suspect this already works for OGM but there are no test.
• let the user access the native NoSQL JDBC-like API and there it's more wild wild west-ish.

As it is we do an adaptor from the JDBC statement API and the Tuple representation in the OgmEntityPersister. We could keep doing that for the custom type and use the JDBC API but that does not offer all the flexibility the underlying NoSQL offers.
The alternative is to offer some generic based UserGridType that accepts whatever underlying NoSQL API is useful per datastore provider. The big problem is that the type would have to be applied not on the Tuple but on whatever Raw underlying type the specific Dialect offers.

As you can see, it requires some experimentation.