Now I'm curious, because I've spent the past little while looking at file encryption headers, whether there's an example where the RSA modulus size is explicitly going to be part of the file header. I guess you could tell even if it was presented as a blob (inside a structured header) because it would be padded to a size that was equal to the length of the modulus...

You don't need to muck around with encryption headers for this. Just install gpg, generate a RSA-1024 key pair, and use the public key to encrypt a rickroll video or something.