Solved. I added a code statement as the first line of my DTL as follows:

 set target = ##class(EnsLib.HL7.Message).ImportFromString(source.SchedulingData)
 set target.DocType = ##class(EnsLib.HL7.Schema).ResolveSchemaTypeToDocType("2.3.1","SIU_S12",tSC)

This loaded my HL7 class with the message from my SQL table field.

Thanks to David for steering me in the right direction.