I suspect the best -current- answer is to mirror, but have the mirrored versions have comments turned off and embedded links that let people comment on the main site.

I'm kinda hoping that the DreamWidth -- with their dreams of lj-but-with-far-better-cross-site-support and separation of trust and watchness -- project gets off the ground and makes following (or granting trust) to someone on an LJ-style site no harder than putting their site and username into your "watch journal" control on your home site.