In one of my earlier projects we had similar scenario to integrate webMethods4.0 with SeeBeyond(eGate Server) to get to SAP.(Bidirectional communication XML-IDOC-XML)
Here we have used HTTP transport that posts (IDOC)from WM to eGate and it will route to SAP system.
We tested and moved it to production which is working as expected.
Sorry i am not a SeeBeyond developer to explain more in detail about this layer.
And this project was implemented couple of yrs back, i dont think there is a SAP adapter released at that time.
But now webMethods has SAP Adapter which can directly communicate with SAP.So it will be easier to handle instead of using another layer seeBeyond.
Regards,