@mpegable: Welcome to the forum
You've got the start of a good and promising codec there. When I tried it on a PAL 720x576 Aviscript Script from MPEG2Dec3.dll I got an error about the input size being incorrect. The actual error was: "Output compressor error: The source image is not acceptable. (error code -2)". The same occurs with both an old and most recent version of VirtualDub.
When I check the "File Information" in VDub the Decompressor for the YV12 is listed as DivX 5.1.1 Codec. Maybe you have a different decompressor acting on yours?
Hope that's some use.
-Nic