I haven't found a way to do that within PC but I'm used to saving the text of PMs to a text file to return to later. I do that either if I run out of steam for the night or if I think I might lose the message before sending -- say, if my browser is acting flaky or the lights are flickering and I have reason to think the power might go out.

I keep Windows Notepad handy on my "desktop" just for occasions like that. I even have a shortcut set up that goes directly to a file called temp.txt, so I don't have to waste time figuring out where to save the file or what to name it. If by any chance I have two things I need to save at the same time, I'll rename the other temp1.txt.