If you have LR active, pressing CMD<ENTER> (Mac) starts a slide-show on the currently selected images, or folder of images. Is that what you're after?
Otherwise, from the Slideshow module, you can export the slideshow as a PDF file and simply double-click on it from anywhere (Desktop or within a folder) for it to start. When you save create it in LR, you have the option of selecting whether it automatically shows in full-screen mode or not. Maybe that'll help?
Thanks Mike. I've run the PDF export, and it's a partial answer to what I was looking for - it would certainly give me a desktop shortcut to the slide show, though I haven't quite figured out how to make it Acrobat loop indefinitely. I'm not crazy about the speed of the PDF export either.
What I'm really looking for is a way to launch the Lightroom program, and have it immediately start a show that I've previously configured, without having to first go to the Library, select the folder(s), then go to the Slideshow and select the templates or options, possibly select specific photos / collections, etc. I'd like to go through that process once, and just save all the slideshow settings.