Because of LJ's...interesting life choices, I've temporarily changed the community's layout and enabled custom comment pages. This allows for subject lines in comments. Please let me know if the current layout is difficult to read-- I'll try to find one a bit easier to read. I'll also do some testing to see if the subject lines show up if you're logged into a journal that has different comment settings.
ETA: Okay, apparently if use the default view for comment pages, the subject heading will just show up as bold font in the comment. Sigh.