Can you add some details on how a producer of this interface would know the values to be assigned here? Does this contain any sort of encoding of generation and Die specific ID in a single UINT64?

Yes, in order to have a generic field regardless of the die type and per-generation changes it will be packed into a single UINT64. Protocol will be installed by silicon code, and consumer will need to be aware of the encoding and decode it. Same applies for stepping id.

Any reason the die specific GUIDs are not declared in the .dec file?

I can't see any problems with that, they will be added in v2 changes.

Regards,
Maciej