Love this example/use-case @Elijah Cotterrell! 
Thanks for sharing

If you want an example of doing this in a wider scope, for a whole package, you can see this example (part of the DeleteHelper sample).

It seems like you might have included in the question a different Transformation than the one the Warning is pointing at?

As it refers to this one:

osuwmc.Scott.FHIR.DemoOutboundHL7Message
(and not: osuwmc.Epic.FHIR.DTL.FHIRResponseToPatient)

And it seems like the Source of that Transformation is expected to be an HL7 message (with a DocType property) but for some reason, at least at runtime, the source is instead of type: osuwmc.Epic.FHIR.DataStructures.PatientSearch.Record

In any case it looks like we're not seeing the wider picture in order to help you address this issue.