Adding read more link to a meta box?

Hi all,
I’m using a custom meta box on a client project. Is there any way to add a “read more” link inside that custom meta box? I know I can automatically truncate the content and then append a link that would take the user to the full post, but I would like for the client to be able to add the link himself, rather than use an arbitrary number of characters or words.

Note: I am not using “excerpt,” just a custom description box. Is there a way to, via theme functions, re-label the “excerpt” box in the “Add Post” UI? Seems like this issue could be solved more easily that way.

Thanks for any help/advice. If there’s a better forum category for this, let me know.