Have been away from healthshare for a few years, so I'll look into what you said and then probably raise a wrc.

many thanks

Many thanks. 

I think I might be able to do a work around along those lines. 

1. replace the base64 entries with a placeholder and save the base64 entries as new QuickStreams. 
2. do the DTL transforms
3. put back the base64 entries just before I call out to the FHIR operations.

Any ways around this? It will be easier for the other developers to keep a like for like payload to OML_021 and ORU_R01 (obvious answer might be use HL7 v2 instead).
But I think I can answer this via attachments in via another API (POST /Binary)

I'll probably still have this need for outgoing payloads to a national system but handcrafting that might work.