You should place a breakpoint in the code that should fork to your marshaller. I don't have the 0.5.0 code here at the moment, but it should be in ActivitiDiagramEditor. There it collects all ExportMarshallers that are active. Your marshaller should be in the list. If it isn't, go from there. Otherwise it should invoke your marshaller.
You're looking at a leftover. The FILENAME_PATTERN used to be used to return a value in the ExportMarshaller#getFilenamePattern() method, so Designer could determine which file to create. That's no longer used, so you don't have to have it. However, I'd consider it a best practice to hold the filename or its pattern in a static member and to just do this:
saveResource(getRelativeURIForDiagram(diagram, FILENAME_PATTERN), bais, this.monitor);
This way you can used placeholders such as ExportMarshaller.PLACEHOLDER_ORIGINAL_FILENAME. The AbstractDiagramWorker class (or in 0.5.0, the AbstractExportMarshaller class) has extensive JavaDoc on this.