Swift: Can I (run-time-checked) look up a CodingKey's corresponding member value?

With a CaseIterable CodingKeys enum and Mirror I can almost implement a runtime-type-checked version of the compiler-magic Encodable implementation. Almost, but not quite, because from a CodingKey value I can't get back to the enum case name, which I need to look up the matching value via Mirror(reflecting: myEncodable).

enum CodingKeys: String, CodingKey {
    case defaultName // default: .stringValue is also the myEncodable member name
    case nonDefaultName = "some-other-name" // .stringValue is not the myEncodable name, no access to "nonDefaultName"
}

I would have expected to be able to find the case name somehow via Mirror(reflecting: CodingKeys.nonDefaultName) but that doesn't seem to be the case.

1. Am I missing something? Is this in fact possible using Mirror? (I'm still on Xcode 9, by the way, faking CaseIterable until I get the real thing.)
2. Any alternative ways to get to the same goal (guarantee that all members mentioned in CodingKeys get looked up on the object and fed to the Encoder)?