'k, so far it works, and with little modifications... but this is changing a value directly in the database.
What I did was make a few modifications in forumdisplay.php and includes/functions_forumdisplay.php, and a change to the 'threadbit' template.
Then setting a thread's 'sticky' field to -1 for a sunk thread. It then shows up as the last post in that forum
So basically all I'd have to do now is to add an option 'Sink thread' to the thread tools menu and some logic to set the field to -1 and packaging the instructions.
This research so far took 15 minutes. Doing the rest will take as much as an hour perhaps (writing the rest of the code and instructions). I'll take care of this after I finish up the v1.2 of my already published hack and reply here with a link to the new hack.