Here are the results of my work today. We now have a transition that uses an Image to filter out the old scene using a "threshold" value. In essence, as the transition progresses, more and more of the old scene is rendered transparent based on a mask image.http://static.stencyl.com/games/29851-0.swf
Any arbitrary image can be used (although I still need to address handling up/down-scaling to fit), including those "Image" types that come from Stencyl itself. As a result, this plays well with the existing blocks in Stencyl. For example, the "image from path" block was used here (with a text attribute) to point to the 'extras' folder where I stored my mask files.
For the examples shown above, I created a few simple gradients in GIMP; you can see a thumbnail of those below.
As you can see, the darker the mask, the sooner it becomes transparent based on the threshold value that gets tweened over the course of the transition.
I'll note that this was inspired by RPGMaker transitions, and a search of the web will find some interesting versions of this; for example:
Presumably you could use those, although it would be up to you to determine the legality of any particular image you find (copyright concerns). Of course, I'll construct a bunch of useful ones to be included, and anyone can make their own easily enough as well!
As always, feedback is welcome!