I would guess you would use a plain Type Object, just like if you were doing dynamic mapping (see my sample on the site).
I spent several days trying to get an Ack back as well doing something similar – with no luck.
Have you tried passing ports between your Started Orchestration? Maybe that would work better to get a message back. Charles Young covers it well here: http://geekswithblogs.net/cyoung/articles/19546.aspx
Might give you some other ideas but you have probably already looked at it