I think I'll see what I can do about community migration; not sure what the API affords, but I know this is a feature people long for.
ETA: The problem with community migration is that it loses the original poster information. It can certainly be done, though, if people don't mind that. Check out the partial mirror of giles_fic_recs I just did at my test IJ asylum. For now what I can do is take a list of communities to archive locally, do that, and think about how to handle migration. Maybe a setting that says "migrate everything" or "migrate only my posts in each community".
And then at some point today today I'll need a netless break to recharge the mental batteries and read the latest Bujold, which sahiya has kindly lent me.