I'll take a look and see if this can be supported.
Having looked at this I've come to the conclusion that trying to support non-primitive, non-primitive-wrapped or non-String Ids in a generic fashion isn't sensible.
So in the new design of SeamSelectItems http://wiki.jboss.org/wiki/Wiki.jsp?page=SeamSelectItemsNewDesign I've made the Entity Converter really easy (I think) to extend to support complex id's - essentially you provide a conversion strategy from complex id object to string and back.
AFAIK @EmbeddedID is only used for 'complex' id's so there is no need to check for it...
Thanks for investigating. The new design looks great, btw.