While there's a lossless H.264, it doesn't fit the implied use-case proposed in the thread. Using lossless H.264 to recompress lossy MPEG-2 would be akin to using PNG to recompress a JPEG -- silly and wasteful.

Sure, it's an inexact analogy because PNG is not a block-based DCT coder, but all lossless H.264 does is set the quantizer as absurdly high as it needs to go to losslessly encode a particular frame. Staring from a compressed source, this is not a good recipe for achieving a space-saving result.