Yes i would suggest changing the icon based on the current state of the window, as it can be opened and closed whether its started in its open or closed state. Same case as for the title bar (bug 16492).
In that case, why not just change the XFCE_APPFINDER_STOCK_EXECUTE and XFCE_APPFINDER_STOCK_FIND macros to different icons? Then, you can incorporate my modifications and still have the new icons.
Kidding I think it's better to have a separate icon I will probably suggest something once we are done with the other icons. It will likely be a variation of the appfinder main icon or something like that
For run, maybe a single gear with a play button in it. I've seen a few like that. The three gears idea works fine if its a large icon, but when its small, it doesn't.
For application finder, a magnifying glass like elementary xfce seems good.