What's there to preview? The <readmore>'d section will show up as a Read more... link, with a linebreak before and after it. You can see the effect, if you really must, by going to CUFP after posting. Or did I miss something?
The point is "after posting"! I'd like to see it during preview as an option. Among other things, if I ask for the effect and don't see it---this provides quick feedback (i.e. I left it out or whatever). Or I might want to place it higher up in the node or such like...
--hsm
"Never try to teach a pig to sing...it wastes your time and it annoys the pig."