I've made a userscript to do this.
Get it at Github.
It gets the latest 5 questions posted on the per-site meta of the current site and adds a diamond to the topbar.
It is red if there are new questions, and white if there aren't.
It decides whether you've seen the questions by saving the latest question via GM_setValue - if the latest question via the API is equal to that, it will show white. So, basically once you click the button, it will go white the next time.
Note: I have no experience with the standard mod diamond so sorry if it doesn't look right!
On sites without a meta, it will always look like the bottom gif above.