While putting together a POC, I wanted one side of my parallel shape to only pick up pdf documents when the file was not marked as an error from the HL7 pipeline marking the message with a parse error:

But how could I access it?

You need to reference the following assembly in your project:

C:\Program Files (x86)\Microsoft BizTalk 2009 Accelerator for HL7\Bin\Microsoft.Solutions.BTAHL7.HL7Schemas.dll

Once you do that, you can add the following filter: