I've been tasked with an interface that needs to supply some additional metadata for the indexing of HL7v2 messages. After toying with a couple of ideas, I thought that it would be useful to subclass EnsLib.HL7.Message, add a %DynamicObject property, and store the metadata in it (it ends up getting sent to the target in JSON form anyway).
And, well, it seems to work just fine ... so far.