I figured I'd be able to use an Index functoid but it doesn't seem to like my first parameter (the scripting functoid that calls the external assembly) - a red X in place of the usual green check mark.
The thing that makes me think it's possible, is that the Index functoid doesn't give me an error at all - it compiles and deploys with no complaints. The problem is that the mapping never takes place, I get a catastrophic failure (IMO) because it doesn't even return an error.
So, any way to use an external assembly that returns a DataTabe/DataRow/DataSet in a BizTalk map?