Hi Steven,
The other solution that comes to mind is to redesign your web service (if that is possible) using a Contract-First approach. You can download and install the WSCF toolkit from this site:
http://www.thinktecture.com/resourcearchive/tools-and-software/wscf
They also have a walkthrough showing you how to design a web service using the toolkit. Basically, when you build such a web service, the WSDL will expose the underlying message schema, and when you consume the web service in your BizTalk project using a web reference, your Reference.map object (in the generated proxy) will expose a Reference.xsd schema, which will show up in your BizTalk map’s list of available schemas.
Hope this helps,
Daniel.