I'm not aware of any forum software that allows a user to do this, but it's a good idea for a future feature, definitely!

In the meantime, I might suggest something along the lines of other people's suggestions here -- compose lengthy replies in some program on your computer (your word processor, your email program, whatever allows you to save and is easiest for you to use). When you're finished with composing, copy and paste the text into the box here. Sorry that's more work than it should be, but hopefully it's something that someone can offer as a feature in forum software in the future.

DocJohn