WordPress is an open source platform which means you can do whatever you want with it, now after that been said, there are a few guidelines when you write a plugin you intend to release to the public which i usually follow and one of theme is
"make the plugin as easy to use a you can."
(this saves a lot of support questions along the way)
So creating something like that should be fine since you are not reinventing the wheel and WordPress users are used to thickboxs and lightboxes added by one plugin to another, just make sure it works right and doesn't conflict with anything else in the admin.
I know that some may say that you should stick to the standard admin UI and design but I for one think that if it looks good and works good, Its GOOD.