It doesn't look possible right now. I've looked through the extension docs (http://developer.apple.com/safari/library/documentation/Tool...), and the overall architecture is very similar to Chrome's. Safari extensions are essentially user scripts with a limited API for access to the UI and user data; if Apple doesn't provide an API hook for a given UI feature, the extension can't touch it. SafariTabs would be possible to implement if Safari fired events when tabs and windows are closed, but it currently doesn't.

Despite every release of Safari reducing the feature set of SafariTabs (which is good), I've had a load of requests for an updated version. It should be available within the next couple of days:

http://stuconnolly.com/blog/safaritabs-and-safari-5/