This may not matter to nearly as many and isn't solving an actual problem, just an inconvenience, so it's not as useful... Now I'm making arguments for why it's not one of the best

Haha, yeah. I can't see myself tackling this one. Usually I'm
for things (no matter how small the improvement). But, I guess something that I've never explicitly stated is that I always try to keep an eye on whether or not suggestions will carry their own weight complexity-wise. That is, if something is a really, really small improvement, then it had better involve a really, really small patch...
Bulking up the forum's code with many things that don't add (at least) equivalent value is just not a smart way to proceed. I mean, in general that's not a smart way to develop (or maintain) software, but in SMF's case specifically, it's especially dangerous considering what a mess the codebase is and how much attack surface is exposed by its technical design.
One thing that would be really cool (and I may work on this at some point), is if SMF had a way for users to configure their own client-side modifications. That way, neither myself nor theymos would be the bottlenecks for suggestions like the one OP is making
and people wouldn't have to mess around with browser extensions. Basically, users would be able to go into their account settings, add a "mod slot", give it a name, maybe adjust the load order (relative to other slots), and then paste some (possibly signed) JavaScript that they'd like to have embedded into their future page loads.