Does AEM provide an actual implementation of the TaskManager API out of the box?  All I can see is the interface.  Anyway our answer ended up being to just write a servlet to change a task's status.

I'm creating an inbox replacement to deal with the massive number of asyncrollout messages we get.  Unfortunately the url the existing inbox uses,. /libs/granite/taskmanager/updatetask , looks like it's restricted to what can call it and throws a 403 Forbidden when called outside of the AEM inboxes....