Dear Jon,
as far is I can remember, we did not use MTOM, because it only works with the WS-RM adapter. This implies, that you have to use the webservice runtime in the backend.
Using "plain" ABAP proxies, this is afaik not possible. We went for base64 encoding using the xsd-type base64binary. It's not a perfect solution, as it blows up the data in average 37%, but it works ;-)...
On the the ERP-side, the data is decoded by, i think a standard class, but i am not totally sure. I would have to ask a colleague.
Hope this helps.
Best regards,
Markus