Designating a dialect for custom metaschemas in 4.18

I'm working with a custom metaschema that is a superset of draft 4, is there/will there be a way to select the draft 4 dialect when creating a Validator class? Currently the create method is choosing the opaque dialect when it fails to recognize our metaschema's id:

https://github.com/python-jsonschema/jsonschema/blob/v4.18.0a1/jsonschema/validators.py#L187

and the referencing package doesn't appear to offer a sanctioned method for registering a new dialect id:

https://github.com/python-jsonschema/referencing/blob/v0.24.4/referencing/jsonschema.py#L544-L556

Is there another way to accomplish this that I'm missing? And if not, are you open to adding something like a default_specification argument to the create method?

By the way, thanks a lot for providing an alpha release, it's super helpful to be able to work through our issues ahead of time.

Start with the create method in jsonschema/validators.py at the linked line, then read the dialect registration code in referencing/jsonschema.py at the linked lines. Review the discussion to determine whether custom metaschemas should select draft 4 or register a new dialect. Done means an agreed, supported way to choose the specification for a custom metaschema.