I sometimes (when I feel unsure getting the formatting right the
first time) just open a fake reply to my own node, with the sole
purpose of having a text area with preview functionality. After
having fiddled around with my update until it looks fine, I then
cut-n-paste the respective snippet into the text area of my existing
to-be-updated node (or rather, into my favorite editor, which I have
attached via Firefox's
It's All Text!
add-on), and just close the dummy reply browser tab.
Anyhow, if you use this workaround, be sure to not inadvertendly
hit the 'submit' button for the dummy preview!
That said, I agree that having built-in preview functionality for node
updates would be nice to have.