I don't think a programatic solution would work well. What I think could work quite well and doesn't seem to be too hard to implement or to be too hard on the server would be to show something like
Your post is kinda long. Please consider adding a <readmore> tag!
on top of the preview page if the text is longer than X characters. That way you get a warning (or information that something like <readmore> exists), but it's up to you whether and where do you add the tag.

Edit by castaway - changed code-tag around the readmore to html-escaped <, >

Edit by castaway: Closed small tag in signature