There is no preconfigured customization option available which would allow you to do this.
However, PostcardViewer-Pro does come with source code so you may be able to modify the code and publish a new SWF file which achieves your goal (to open Lightroom's .exe file using ActionScript from within a browser).
Please see here for details: http://simpleviewer.net/postcardviewer/ … rt/#source
However, a gallery which functions as you request would likely have a limited audience as I suspect more potential viewers of your gallery would not have Lightroom than would.