My boss has been wanting something like this for months now. Thank you so much for providing this awesome hack!
I do have two issues though, and they both pertain to adding spell check to quickedit. The statements are set up properly in SHOWTHREAD, but when I click the Spell Check button, the window loads up and I promptly get the "Spell Check complete" message, despite there being obvious misspellings in my test message. What's even worse is that if I use a test message with no errors with quickedit, the spell checker will REPLACE the entire contents of my test message with the default "enable Quick Reply" message.
It works great everywhere else, but I'd love to see a fix for these two bugs since quickedit is a place I'm going to want to have this used regularly.
(I should add, I am getting these bugs with the most recent version of Firefox.)