I like to have separate sites in Google Analytics most of the time, but sometimes it would be helpful to watch all sites/ domains at once, to spot peaks, just so you're alerted in case one of the sites is flooded with traffic for some reason or other ('cause in these cases, you often find yourself needing to adjust the site, e.g. its caching, or post an explanation to new users).

Does anyone know if this is possible... ideally without having to change the individual tracking codes in all of my different templates?

I don't have an answer, but do the stats update quickly enough to allow you to notice a spike while it is still in progress? I was under the impression that it updates every few hours, so you may not get the notification in time.

I use analytics, but primarily for analyzing long-term trends so I don't know how accurate it is in the short-term.