Probably the simplest way would be to use the java apis inside a portlet that is deployed to MWS.
For example:
String thingid_string = "/meta/default/user/0000005284";
com.webmethods.portal.system.IURI principalURI = com.webmethods.portal.system.PortalSystem.getPortalSystem().acquireURI(thingid_string);
com.webmethods.caf.faces.data.dir.IPrincipalProvider principal =
com.webmethods.caf.faces.data.dir.PrincipalModelFactory.createPrincipalModel(principalURI);
String userID = principal.getPrincipalID();
#webMethods-BPMS#webMethods#MWS-CAF-Task-Engine