Hello,
I still don't see why you can't do this the normal way in Workflow. Do you have UWL? Does the approver receive a workitem?
But anyway, SAP_WAPI_WRITE_CONTAINER writes to the container, as you know, and I'm sure it can write a multiline text element (as I said, I have never had a need for that function module) -which is what rejection comments are.
Please show what you've tried. The simplest thing to do is write a small Z report to write to any active workflow in your DEV system, to test out what works.
Again: are you sure you can't do this with regular workflow? Why is your situation so different?
regards
Rick Bakker / hanabi technology