Since you requested this, maybe you can help. The hurdle to implementing this is not technical (I can easily loop over the profiles and check for notifications in the background).
The hurdle is how to display them for the non-active accounts.
Any suggestions that won't make the notifications menu look like a CVS receipt? Keep in mind, notifications include reports (if you're a mod or admin) as well as registration applications (if you're an admin).
The other question is how many accounts do people normally have at once? I juggle three (this one, my LW alt, and a non-privileged local test user), and I feel like that's a lot.
Would the first three accounts be sufficient? Accounts can be re-ordered as needed for this to work.