I've taken to using a simple server on a public IP that serves different JS based on the referrer, and a bookmark for inserting a script tag in the current page.
It doesn't auto run, but it is easy test, install on a new system (just a bookmark!), and extend with extra behavior that can be centralized. Additionally, an update immediately affects all clients.
It doesn't auto run, but it is easy test, install on a new system (just a bookmark!), and extend with extra behavior that can be centralized. Additionally, an update immediately affects all clients.