7
           
            
                
     
    
    So I looked into it, and the issue is that when the Website is closed, XOOPS goes to 
 /site_closed.php  
and from there it calls the 
 system_siteclosed.tpl  
that is located in the current theme, e.g.:
/themes/
xswatch/modules/system/
So there would be a lot of hacking involved, starting in 
 /include/common.php  
in line 325:
 if ($xoopsConfig['closesite'] == 1) { 
    include_once $xoops->path('include/site-closed.php'); 
}  
to redirect to your "site_closed.php", or you can changed the "site_closed.php" file itself.
Because of the constant hacking that you would have to do for each new version of XOOPS, it might be easier to simply create a new "mobile" theme, and use that?