As Ingmar has mentioned this would probably be possible with a plugin but as far as ExpressionEngine stands at the moment it leaves those kind of operations down to whatever you as the programmer place into your templates. I really really like this fact that ExpressionEngine makes no assumptions as to what you want in your templates and the fact that it totally abstracts the data from the template side of things is just in my mind the most powerful feature of EE.
What I like to do on sites also is have a simple conditional surrounding my read more links so that if there isn’t anything more to read then there isn’t a link shown. Something like this :
{if extended != ""}
<a href="http://{url_title_path=" title="Read - {title}">Read more…</a>
{/if}
This is a great feature to add into a site as it literally only takes a second to do but makes your sites much more easier for people to navigate around. I personally don’t like clicking on Read more… links that just take you to a page that has the exact same text on that you just read as it kind of defeats the object of the link in the first place 😉
As Ingmar said though you could probably do something with a plugin although I think that it would probably be the sort of plugin that would be quite specific to your own setup and may not work on all sites without a bit of modification.
Best wishes,
Mark