How can I patch pmdevtopic?
OK, so, you don't patch the nodetype itself, you patch the htmlcode and other code-type nodes associated with the nodetype.
Under Relevant pages are listed the htmlpage nodes which render various views of any node of the given nodetype.
However, I don't think you'll need to alter (or add) any views of usergroupnote nodes.
At this point, aren't we just talking about modifying handle_threaded_nodes (which is the htmlcode behind Recently Active Threads)?
I reckon we are the only monastery ever to have a dungeon stuffed with 16,