When I'm viewing a web page in one browser, I'd love to be able to hold down a modifier key on my keyboard before clicking a link, in order to open that link in a specified browser. (It would be best if the browser was chosen based on the modifier key held down.)
Basically, no. There is no trigger that would detect that (except maybe detecting all clicks, and that is unlikely to be useful) and no way to know what the link would be to.
A commonly used macro simply takes the current URL (FrontBrowserURL token) and opens it in a specific browser (perhaps dependent on the current browser). So in Safari, it opens in Chrome, and vice versa.
While you can't use a modifier key with a click to trigger a macro, you could use any other KM Trigger , like a HotKey, to trigger a macro that gets the URL under the mouse, and then open it in the browser of your choice,