I've made a userscript to do this.
Update: This has been added to the dev version of my SE Additional Optional Features userscriptmy SE Additional Optional Features userscript - please install that instead of this. Any updates will go straight to that userscript, not 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.
V1.1:
- added support for multiple sites (I forgot about the possibility of using multiple sites last time!)
- made it so the box closes if you click outside it
- made it work in firefox (thanks @Brock :)
v1.2:
- genuine colours (thanks @Doorknob)