Old post, but I have the same question. Or at least: I think I have…
Once you have used a custom field it stays in the dropdownbox. But its only the KEY that WordPress remembers. It could be cool if there was a way to make the VALUE-field stay aswell.
And let me explain why: I have a theme that uses custom Fields to show an image on the frontpage. But since I only have about 10 different images (More like icons really) that I use for my theme, it could be cool, if I could just choose them directly in the custom fields without remembering the URL for the image.
Is there a way to do this?