Actually it helps to sleep on a problem...
Yesterday I was digging into the code and hitting walls everywhere, when today I realized the solution was right there the whole time!
Well, it's a stupid, lazy solution, but still: Just use a Temp Paster object that's bigger than your sprite, For each sprite paste the sprite into the Temp Paster, apply the shader effects to that and paste that into your main Paster object. Voila!

Reduced the resolution on purpose, so we can see it working. Works well enough for my needs. Thanks again for making such a great plugin.