I'm trying to add a Processor to this MSG preset that will scale the entire image, giving the effect of "zooming out/back" and showing more of the image on the canvas. I added the Scale proc at the end of the chain and set all the R/G/B inputs to R/G/B outputs, but the scale has no effect at all. Must be another way - this is something I need to do often, so are there some general tricks for accomplishing this technique? Thanks for any help.
You can use the interactive warp effect using one of the scale options with keyframe animation to scale in-out over time.
So, i looked at your Scale preset. The IO routing for the last processor is not correct, note that you are trying to route the B output to the Float Img bus stream.
The other thing to be aware of involves recursive IO routing when working with geometric effects and their associated processors. If you don't want the wild and weird visual effects you get by recursively routing IO with something like the Scale processor, then you'd want to route the LABtoRGB output to something like tmp Rimg, tmp Gimg, tmp Bimg, and then use those as the inputs for the 3CScale processor, with the correct ROut, GOut, BOUt for the output of that processor.
Here's a tip that discusses recursive routing effects. You should read this in full to understand what i'm talking about above.