There's a mod here that does this:
https://vborg.vbsupport.ru/showthread.php?t=266744 but the free version only does it for the CODE tag, and it looks like maybe the premium version isn't being sold any more? At least I couldn't find a link to it, or find it on his site. (ETA: he has in fact announced on his site that he is no longer offering premium vbulletin mods.)
If you can do some coding you might be able to figure out how it works and write your own.
The javascript idea might work, unless you really want to keep some people from seeing the content, because people will figure out to disable javascript or just view the html source.
I think it's not too hard to provide php to handle a bbcode tag, but you also have to make sure the result doesn't get cached (I think I asked nhawk about that in his mod thread). Edit: does vb3 cache posts or is that just vb4? I don't remember.