Way to add an additional context to a Base Registry error?

Message objects do allow for OEM extensions. There's the typical "Oem" object defined at the same level as MessageId and Message here: github.com/DMTF/Redfish-Publications/blob/main/csdl/Message_v1.xml#L88

You are also allowed to modify "Resolution" in your usage of messages. There's no requirement to take the resolution from the message registry as-is since DMTF can't give implementation-specific information for every possible situation. We have this statement in the Resolution property that describes this:

> Services can replace the resolution defined in the message registry with a more specific resolution in message payloads.

"ActionNotSupported" makes sense to me for what you're describing; you can certainly provide a resolution like "Enable feature X on resource Y." in your message object to convey that situation back to the user.