Could you have it show "last active in any way" (the date you use for the "active in any way" site-wide statistic) instead of "last entry posted in this journal"? That seems like it would solve the major part of OP's privacy problem, and maybe even be more useful for assessing how active someone is on the site (since many people are more active in comments and communities than own-journal posts anyway.)

ETA: Although I can see how that might cause a different privacy issue - of the "my stalker/abusive SO knows whenever I do anything" type. (Although people in that situation would probably not like their stalker to be able to tell they've made a locked post, either.)