I wouldn't use a DTL for process flow control.  I would have a BPL make those decisions and use the DTL strictly for transformation of the message.  The BPL diagram can then just fall through to the end once the REST call is successful.  The operation doing the REST call should then handle the infinite retry, Failure Timeout of -1 would tell it to never stop retrying as you said.

HealthShare Health Connect would work, which I believe you have, Scott?

Take a look at the EnsLib.HL7.Message class.  It has the following method to import from %String:

classmethod ImportFromString(pString As %String, Output pStatus As %Status, ByRef pConfigItem As %String, pIOFormatClassname As %String) as EnsLib.HL7.Message

It also provides similar methods for importing from streams, which could be a file character stream.  You can view all of these methods using the class reference from the documentation.  Hope that helps.