if I remember right here is what I did:
retrieve all selectItems. Check if the value matched one of the items, and if it did, set the value in the valuebinding of the selectOneMenu component ...
TheSid's solution should work.
However, this was an oversight in the JSFClientSession API. There should be a single API call for this. I'll make sure that this is easy to do in the new HtmlUnit-based JSFClientSessionAPI.