Filter to Create JPEG Compression Artifacts?

I started out looking for a way to do this in Photoshop -- and though I found a number of other people trying to do the same, I didn't find any solutions except "save as JPEG." Which isn't exactly a convenient method if, say, you're trying to match compression artifacts between content on multiple layers.

My ideal JPEG artifact script would have a slider for JPEG quality and also allow comparison with a second image by overlaying and masking the comparison image, then manually setting the locations of the mask's sides. (See the attachment for an illustration of how this would work.)

Would it be possible to do this in FilterForge? I have zero experience with creating filters, but at a glance through the available components, I don't see anything that looks like it could do this easily. Am I missing something?

Or perhaps it could be done via scripting? I've never used LUA or any C-esque languages, but I did find libjpeg, which is a LUA "ffi binding for the libjpeg 6.2 API." I guess using the libjpeg library would make it an "unsafe script," but with that caveat, could this work?

Like I said, I've never done any filter-making or LUA scripting before, but if this idea seems possible, and if someone could point me toward a good starting point, I'm game to give it a go. Unless someone with more experience than me wants to take on the project, of course!