Ian, Spuds is just testing how well it works without doing that, since mroz and I both got good results with it - your solution is probably the most elegant, now that it's been proven to fail.
MFA, shared memory sections = race conditions, which is no better than we have now. Everything has to be made threadsafe at either the plugin or the scripthost level anyway.
Fizick, as long as you're here
have you ever thought of creating the idx clip by cloning vi with x2 or x4 height/width and requesting blank frames from this new clip? This way it will be cached and can be limited by setmemorymax - the downside being that if it ever gets limited, it'll drop performance, but not nearly as much as swapping to disk. Since it's a global, it may have lifetime issues that using it in a single class doesn't, I don't know nearly enough about avisynth's smart pointers to know.