You need to use a Correlation Set to promote the properties when you send a message out of the Orchestration using Direct Binding. It seems a little odd, but it is the only way to accomplish what you want.
Just define a correlation set with your property in it and initialize it on the Send Port. You will not have anyone following the correlation set.
Charles Young talks more about it here: http://geekswithblogs.net/cyoung/articles/17364.aspx or http://geekswithblogs.net/cyoung/articles/19546.aspx