I know the GUID string for the device I want to communicate with. However, I can't figure out how to declare or cast the string in such a way that I don't get RPC_S_INVALID_STRING_UUID as a return ...

GUID (or UUID) is an acronym for ‘Globally Unique Identifier’ (or ‘Universally Unique Identifier’). It is a 128-bit integer number used to identify resources.

universally unique identifier (UUID). GUIDs are 128-bit globally unique identifiers that are automatically generated based on close to two zillion frequently varying factors.

When you see an UUID, the first number of the third set specifies which version it is. So, 589bc816-502f-11ed-b8f4-18c04d808bd7 is a version 1 UUID.