How to set the default index/landing page on UCM & URM with an SSO configuration

I''ve got UCM and URM 11gR1 PS4 (on weblogic server 10.3.5) configured for SSO via IBM WebSEAL.  UCM is behaving fine, in that the users land on the home page (idcplg?IdcService=GET_DOC_PAGE&Action=GetTemplatePage&Page=HOME_PAGE) after negotiating WebSEAL, however URM puts up the guest home page.  This is mainly annoying, as the users have to click something on the guest page to get to anywhere useful.  I think that the difference in behaviour is because, for technical reasons I won't go into, the URM junction is transparent, which means that the webroot (urm) has to be used, whereas the UCM junctions are standard, which