Add Reminder Text to Add New Posts form

Anyone know of a plugin, or describe (in detail) a hack so that I can add a reminder to my authors (multi-author urban planning blog) on the Add New Posts admin page? I want to add text above the tool bar and above or below the title area that says something like the following: "For posts longer than 100 words, please remember to click on the page break button ("Insert More Tag") before the100th word."

(Yeah, I know I could set automatic excerpts but I like the manual "more" tags better.)