I found that even when PEOPLE_USE_EXTENSIONS is enabled, there is no way to add delegates to the controls created before the extensions were loaded.

The last lines of appg/init_people.php should be changed to:// INCLUDE EXTENSION
if ($Configuration['PEOPLE_USE_EXTENSIONS']) {
include($Configuration['APPLICATION_PATH'].'conf/extensions.php');

// Make sure to get all delegates from the extensions into objects which were
// constructed before the extensions were loaded.
$Head->GetDelegatesFromContext();
$Banner->GetDelegatesFromContext();
$Foot->GetDelegatesFromContext();
$PageEnd->GetDelegatesFromContext();
}
?>

That should be enough to allow extensions to work with full functionality on people pages.

Also fixed in SVN: http://lussumo.com/bugs/discussion/76/