Are you dealing with multiple DocTypes and Categories of messages going to Epic, or are all messages the same schema?

I know you're not crazy about a custom operation, but if you take a look at the source for EnsLib.HL7.Operation.TCPOperation, you'll see that it would be dead simple to copy/extend it, add a check for the population of the Ordering Provider field, and the logic to populate it with a default if it's empty.

From within your task, you can obtain the task's classname with $CLASSNAME() and query the %SYS.Task table using the TaskClass of your task to fetch the OutputFilename column.

If you want to use that file under your direct control, you can set "Open output file when task is running" to "yes," enter the full path of the filename, then set the previous setting back to "no." The filename will remain in the table.

If you're calling the same class under multiple Task schedules or with different configurations, the schedule values and settings are also available to help you refine your selection. Settings are stored in $LIST() format.

EDIT: You can also define a Filename property in the task class, assuming it's a custom class. It will show up as a configurable field in the task editor. That way you don't have to deal with the OutputFilename settings.