To add a little more context, REST web services typically involve posting an HTTP method such as GET or POST on a given object. The URI would have the given object specified in which the change is being applied as part of the syntax, ie: http://SERVER1:8051/caisd-rest/chg would apply an update on change orders. As a result, SOAP methods such as callServerMethod do not necessarily translate into REST directly.
My feeling is that one could indirectly trigger the spel method you're interested in by leveraging the given object that the spel method might be attached to at the factory level or via an alternate approach such as invoking an event.