@Honeyko: The predefined PARs are stored as 4 bits in the VOL, custom PARs use an additional 16 bits (8 width, 8 height). So in certain cases it is possible to modify the file in-place, but not always. Of course it's not necessary for it to be written in .NET, but C# is the language I'm most comfortable with. MPEG4 Modifier is something I did mostly for fun, I didn't really even need it, and for me writing it in C++ would have been more frustrating than anything and I wouldn't have written it at all.
@squid_80: