Guy, I can't help with your problem. In fact the crop in CS6 reverts back to the image's original aspect ratio if you try and view full screen with it set to 16:9. But I was going to ask if you had considered scan & pan with your slides. I've used Premeiere Pro to mix stills and HD video, and find the stills take on a dynamic quality if zoomed in/out or panned. You have to be very slow - I reckoned on zooming from 2200 to 1920 in five seconds. If you disolve between stills, you need to kep the movement happening into the disolve, and the best waY I found to do that was to place the stills on alternate video tracks, and overlap them by the length of the disolve, and place a disolve on the end of one tract, and beginning of the next.
To show a 16:9 image full screen you screen needs to have a 16:9 aspect ratio display. There are many 1920x1080 computer displays that have a 16:9 aspect ratio. Most computer displays do not have a 16:9 aspect ratio. They have 4:3 , 5:4, and 16:10 or 8:5 aspect ratios. If your display is not 16:9 there will be borders. If you do not want to automate your process with an action you could write a script to automate the process.
Because the image composition will change so much when you crop a 3:2 aspect image to a 16:9 aspect ratio. I normally make that process interactive so I can choose the crop composition. I would even suggest not cropping portrait aspect ration images to a 16:9 landscape aspect ratio.
Thanks for the suggestions. I probably should had added that I am only generating mock-ups at the moment. After I scan and edit a slide, occasionally I like to see how it might look at 16:9, that's all. It is going to be a problem transferring slides to HD, more so that my original audio-visuals are actually on three screens. But that's a problem for later. At the moment I was hoping there was a quick and easy way to see a 16:9 mock-up.
Trevor, it's interesting that you mentioned Premiere, because that's what I'll be using for my audio-visual transfers. Since you clearly have some experience with Premiere, you might like to jump across to the Premiere forums and make comment on my recent post: http://forums.adobe.com/message/5404238#5404238