Definitely. I did this not long ago but used a link in the avatar section instead. Using a button wouldn't be that much more difficult at all, I however prefer the link so that it doesn't pinch those on low-res displays and create a slidebar...
http://www.mameworld.info/ubbthreads/sho...;o=&fpart=1 The link will vary somewhat depending on the threads version used however.
I discovered afterwards that there's already a mod for this as well, where the PM link is included in the 'post extras' section of the post in the form of a tiny hyperlinked gif.