Well I don't think there is anything official about avisynth. It's just a bunch of guys that work together on the program and it's filters. If you go to the avisynth development forum the guy that made vsfilter probably is there and they all exchange information freely so it shouldn't be a problem.

I don't think you need a custom dll made though you could probably accomplish this with a script or a function in avisynth. I've never used vsfilter but i'll take a look at it. If I see a way to do it i'll let you know. I'd still go to the development forum and see what they've got to say about it though.