Two other ideas I have on my mind currently:
1) For each custom-locked entry created, automatically generate a private memory, using the default title pattern, with a property assigned to it of a form something like "CustomSecurity#filter." for each filter that's in use. Such properties would not be available in the usual memory-property select-box, and trying to enter a property in that format in the new-property box would result in an error. The separator -- a hash sign, here -- could intentionally be chosen to be something that has already been set to throw an error when entered in the new-property box, to ensure that nobody's old properties would get trampled. Maybe the non-breaking-space (U+00A1)? That's pretty hard to enter from your keyboard at all. :-)
2) Add a third security icon to be displayed on custom-locked posts. Maybe a gold lock, instead of the usual gray? This would be another (and simpler) way to help easily locate such posts. To some extent, it would alleviate any need to run a expensive (in server and admin work) backfill to capture old entries under the system above; users could individually use the month view to page through their journal and find custom-locked entries. On the downside, you'd probably want to give users the ability to control whether the icon was used, at least at the level of "only show me the gold lock" versus "show anyone who can see a gold-lock post the gold lock".