Doxygen has the \todo command which produces documentation that shows up in the function it resides in and also gets aggregated into a global Todo List document.
Can this behavior be duplicated or simulated for arbitrary other tags? I would like to do something like "\tag XXX" and the have that comment aggregated into a global "XXX List" document.
Yes you can create your own tag lists. See http://www.doxygen.nl/manual/commands.html#cmdxrefitem.