I discovered the existence of the incredibly useful local/ folder and useful locallib hooks earlier today after seeing Tim's Commit to xmldb. This was quite frustrating as i've written modifications which have required DB table creation and really could've used that hook point!
So in the spirit of preventing others from suffering the same fate as me, i've (somewhat poorly) started Development:Local_customisation in the wiki. Could developers who know about these local hooks contribute so other poor souls don't need to write my_hacky_function() into db/upgrade.php too ;)
Dan