Character limit question: No. It's a global property, changing it somewhere changes it everywhere. Besides, if you need more than 20,000 characters, you need to stop, step away and take a deep breath. Preferably from the tailpipe of a 1979 Buick Century with a bad catalytic converter.

Custom bbcode for strikethrough: please die! sure, why not? But if you want a button for it, someone's going to have to make a 21x20 gif of struck-out text, because I'm more than a little uninterested in doing so. Use the other images on the edit bar as examples.

Just ... try not to overuse it too much, k? I don't want to have to take it back out and leave your posts littered with little [s][/s] tags.