I believe having custom currents would both be easy to implement and beneficial to everyone. Have a maximum of like two customs or perhaps even just one per post.
The only thing that would need to be done is two new rows in the DB; one for current name and one for current content. It'd be fairly easy to read, too -- if the name data is empty, then ignore it, same as if the music or mood data is empty.
I know it's usually frowned upon to add things to the DB unnecessarily, but I believe this feature would benefit everyone on LiveJournal, for the rare times it might be applicable.
Have you ever typed Music: Blahblah TV show on TV? I personally haven't, simply because my television is in the other room, but I've seen my friends do this many times.
The only disadvantages would be a potentially larger DB, but one would argue that it would actually save space, as typing "TV Show" as name and "Friends" as data, for example, would be shorter than typing in the Music field "watching Friends on TV". Though the added space in the DB is rather nominal, it very well may be an issue.
It'd just be easier to be able to do something like that, in my opinion. Comments? Suggestions? Public flogging? I'm open to ideas. ;o)