Then he can wait for the RNG to produce this same random data. Eventually it will produce a file which matches. A dynamic solution of sorts, because he would have to be quick to diff, before the file starts changing again.

I feel that the compressor for a true random stream is a true random generator. If I quickly show you a screen of black-and-white unpredictable noise, and ask you what it was, you'd compress/understand/recall that as "generate_noise()". I do not feel that this is lossy compression, for what did you lose? The ordering of a random file? Random files have no order to lose.