I have a plug-in that handles its own url schema (something like mystuf:// ). If a user clicks on such a link when the plug-in isn't installed (s)he gets an ugly error. So I extended my installer to write into the Registry (for Internet Explorer) and the user.js (for Firefox) to ADD a custom string to the user agent string. These additions survive a browser upgrade nicely (Microsoft uses this technique to indicate the presence of the dotNet Framework).

Now I need to extend this to Apple Safari and Google Chrome on all supported platforms.

To be clear: I'm not looking how to REPLACE the user agent string, but how to amend it with additional information.

Any hint?