Additionally, if you post in a community using the "Private" security option, you will not be able to see that entry from the LiveJournal website, nor will the community members or the community maintainer while logged into the maintainer account. The maintainer will be able to see it if he/she logs on as the community account. Some downloaded clients may also show the entry in the community history.
Only problem I see is for people who already posted privately into a community; I don't know how many people have done this, but I don't want to take this out without people giving the go-ahead, lest anyone have a creature of the bovine nature. ;-)