It puts somewhat more load on the server, but how about detecting readmore tags during preview, and displaying a comment below the input box, to the effect of:

I see you're using <readmore>. You won't detect its effect in this display, but your message will be shorter in the summary lists.

OK, maybe something less pompous than that.

Yes, it adds some processing, but is it much more than the current test to see whether you have any text outside the signature tags?