Literally minutes after publishing my post on calling a WCF Web Service from the SSIS Web Service task, I checked back with the forum post that I'd mentioned in that blogpost http://forums.microsoft.com/TechNet/ShowPost.aspx?PostID=1023821&SiteID=17 only to discover that Uwe Heinkel had given me the peice of information I was missing in the first place. So the crux is I don't have to hand craft the dtsx xml file to get it to play ball.
I simply remove the following line from the wsdl file
<xsd:import schemaLocation="http://localhost:50344/MyWebService/Service.svc?xsd=xsd2" namespace="http://schemas.datacontract.org/2004/07/"/>
and it all just works as expected.
damn.... I was having soooo much fun wading around in the dtsx xml file.