charm almost works; once you post, it archives your post to .ljarchive/postname, and outputs it, and then you can crosspost from the command line with

$charm -u USERNAME -q -x ARCHIVEFILE

In practice, I have to use

$charm -f CONFIGFILE -q -x ARCHIVEFILE

because I haven't managed to figure out how to get my accounts to share a config file peaceably, but it's still much easier than the next-most easy.