Although Mantis can keep notes regarding specific issues, it cannot securely track day to day logging of individual developers. A solution might be to create a developer specific issue that would never close, but this is somewhat clumsy. Another solution would be to just hold the journal within the wiki, but this has security issues, and doesn't allow for the ability of developers and managers to comment easily on notes as they can within the issue notes. I believe the better solution would be to add journal interface to mantis. There's a couple of ways this can be done as well. Natively writing it in, or similar to the wiki plugin, using an external blog project as a plugin. A colleague has integrated forum with mantis, but from what I can tell it is not configured for privacy, or doesn't have that capability. I invite him to comment if I'm mistaken.
JOURNAL for the table mantis_bugnote_tablemantis_bug_table and mantis_user_table$g_journal indicate whether or not to allow the journal$g_journal_threshold indicates the user level allowed to keep a journal