One simple heuristic , if the OP managed to use code tags, then readmore tags should be automatic (around the code); hounding newbies to add readmore tags is waste of time; Do we need to get "in your face" about readmore-tags?

As for coding this enhancement, its trivial, see Re: Proper nesting of HTML to be enforced (the code) / tagApprove / perlmonks approved html parser, Re^2: metacpan link shortcut?

More nodes on this type of thing, you can certainly go far with bookmarklets or a Free Nodelet hack
Re: Approval feedback
Re: Approval feedback (Gentle Feedback Free Nodelet Hack)
Re^2: Put some of the posting guidelines directly above the posting form:
<readmore> for [Anonymous Monk] renders with a background, how about only an outline?
Free Nodelet Hack: a graphical editor for composing nodes
Re^2: create link from my image
themes for Anonymous Monk or try out themes without changing display settings, Re: CSS Show and Tell: Colored Code (tag your themes, theme design tool)