I thought I heard that some file formats have "headers" that go at the end of the file. I think a demo of this was a file that was both a zip and a PNG or something. If I remembered right, you should be able to make a similar hack here.
If the only headers are at the end, then yes, that's a really neat idea. I'm not sure of the constraints for zip files. Maybe it would be interesting to brainstorm with some people with a lot of file format knowledge to find additional such formats. But if the formats have any constraints at all on the first few hundred bytes, those generally won't be satisfied by the prefix here.
