I'm not sure, but I think that's a better question and I think you would have to run it every so often to catch the newer users, right? Or you could make/commission a basic add-on that would also set this option automatically for all new users who register, in a similar way as setting your forum default to auto-subscribe users to the threads they post in.
Also, in the non-technical side of things, what about users who don't want to watch the resource category? Would they be able to opt out? Maybe you could somehow tie watching the category to a usergroup promotion?