GroupFooter page

Is there a way to create support for a GroupFooter page and a [[nogroupfooter]] 
directive? It's obvious how to do this by modifying pmwiki itself, but not so 
obvious (to me, anyway) how to do it as a local customisation.

I want to create << previous | next >> alphabetic links based on a sort of 
RecentChanges. I can do it in a local/Group.php script that modifies 
$PageFooterFmt; I can do it as a $DoubleBrackets entry to insert a [[navbar]] 
(or some such); not so obvious how to do it as part of an author-controlled 
GroupFooter.

Thanks.
-- 
John Rankin