Well I don't know if the mods will add what you want, but in the mean time I have a suggestion. Open another browser window and hit post new thread on any open forum. Copy and paste your post into the new message box then hit spell check. Once you have made corrections copy and paste the results in the old message window and close the new one. I know it's a lot of steps, and not really a solution, but it works for me as I am probably the worst speller here.
Haha just did a spell check and no mistakes. Wonders never cease.