I highly doubt this is possible, but I could swear I've heard of it somewhere.

Is it possible to put an actual image link in your external stylesheet? Or, could you put like an entire set of HTML in there.

For example, let's say I want to add a "Recent Additions" menu on the right side, and the usual left nav menu. Would it be possible to somehow link to an external "Recent Additions" menu, that you could change in one stylesheet that would change on every page. This would of course eliminate the painstaking task of going through every page and editing the code. On dial-up.

I know it's far-fetched, or maybe I'm just oblivious and it isn't. Anything?

you wouldn't be able to change the content but only the look. However if you use php includes you would be able to do that.

to build on rosey's answer...

css pages contain only css coding for formatting the appearance of an area.

for actual coding and stuff, php or ssi includes will be the way to go (or iframes depending on the situation).