I've gone ahead and flipped a long-requested config change for
DismissableSiteNotice (which controls local, not Central, site notices) to
allow anons to dismiss them.
IIRC the only reason the dismiss wasn't allowed for anons was that back in
2007 or so it would have been prohibitively expensive to deal with more
cookies, especially since I think local sitenotice was still being used for
fundraising banners and such.
My understanding of our current cookie-handling config in the caching layer
is that this shouldn't explode. But if it does, or it doesn't seem to be
working right, deployers please feel free to revert it and we'll poke at it
some more until it works better -- it's an easy config switch, just revert
and sync the settings file.
If there's a better process I should follow on this sort of thing do let me
know, I don't mean to step on any toes!