I had documented a workaround at http://blogs.msdn.com/b/mdoctor/archive/2009/09/08/using-one-way-operations-with-the-wcf-adapter-in-biztalk.aspx a while ago.

This post is to inform you all that the workaround has been incorporated into the WCF Adapter product code. Henceforth, it will be possible to set up a 1-way Send Port which uses a binding such as BasicHttp (or the WCF-BasicHttp adapter) and invoke a 1-way operation on an external service. This feature will be present in the next version of BizTalk scheduled for release in the first half of 2013 (a CTP of which will be available within the next few weeks).