By default spring security will forward the original request after successful login. You can prevent that by adding the following to Config.groovy:
grails.plugins.springsecurity.successHandler.alwaysUseDefault = true
grails.plugins.springsecurity.successHandler.alwaysUseDefaultTargetUrl = true
grails.plugins.springsecurity.successHandler.defaultTargetUrl = '/my/default/url'
The two properties sound like they should do the same, but in my case it only works when I have them both. Perhaps you can remove one of them and get it to work...
This will alter the behavior any time you log in, not just after a timeout. If you need a more dynamic solution, I guess you have to dig into the spring security source code...