You can fairly easily with a post-processor material specified in a post volume/camera in-editor. I think what the Lens Distortion/Composure plugins do is just setup shaders for a material, and then their effects serve largely as the basis for those. And you can insert those materials into one of these four stages: before tonemapping, after tonemapping, replace tonemapper, or before translucency.
What, however, is not possible from something like an engine plugin (even with some minor engine modifications to add injection points into the post stack)… As I discovered earlier this week is doing something similarly solely from C++. THere’s just no real good way (seemingly) to modify the post-processing stack in any meaningful way from code. I was hoping I could do a plugin as a nice, standalone engine plugin that could be easily distributed to folks, but alas.
So, depending on what you want to do and how you want to approach it: if you’re only working within the editor, you can use post processor materials (and take a look at the Composure/Lens DIstortion plugins, as they’re editor-oriented as well). If you’re working within the engine/C++, then your only solid bet that I’ve found is to just go in and modify the stack itself. And doing that is, actually, really easy. It’s just unfortunate there’s no standalone/external/distributable way to modify it.