Referring to the attached picture,
1-Is it allowed to define such a DRF in the ASME standard?
2-If it is, how should we materialize datum B for inspection?
If it's a surface - it can be a datum. This works as defined. Datum C can be simulated with two collapsing surfaces that are parallel to the engineering drawing graphics as specified. A CMM would make easy work defining the specified datum.
Ultimately the datum plane C is resolved into a plane that is perpendicular to Datums B and A too complete the three dimensional coordinate system or DRF. The derived datum C from the simulator is rotated to achieve the correct DRF orientation.
Tell me and I forget. Teach me and I remember. Involve me and I learn.