While I share the dismay of previous respondents with the way some posters ignore the helpful admonitions on the preview page, I do think a variant of OP's specific suggestion re automating <readmore> tags may have some merit.

But rather than merely providing another message (to be brushed by), could we not do a line count on the draft when "preview" is called and -- when that count exceeds whatever we set as a threshhold("50|100|150?") -- insert <readmore>:

• After the last line terminated by a period-and-new-line or <br> or </p>
• at the transition from narative to <code>
• after the last previous </code>

and a </readmore> after the last line of the post.

It seems to me likely that the typical offender is new to PM, and thus is unlikely to have used User Settings to turn the preview default to "off." Clearly, relying on that assumption would NOT assure us that all very long posts benefit from <readmore> tags and -- equally clearly -- those posts in leetspeak and/or without markup might slip by, but force-inserting <readmore> might be a start, if (ah, here's the real stinker!) not to difficult to implement.