As a Computer Applications teacher, I have students turn in a lot of work online. Some of the tools I use, such as Blogger and Thimble have different URLs (and domains) for their public-facing links that can be turned in.
On a non-trivial amount of times, I've had to comment back to students that they've either submitted their editor link or a preview link for Blogger or the editor link to their Thimble project.
As an example, Blogger editor links will contain blogger.com meanwhile public posts will be under a *.blogspot.com subdomain and Thimble projects are edited on thimble.mozilla.org but published links are at thimbleprojects.org.
I would like to see Canvas provide a way to restrict URL uploads to a particular website/domain in a way similar to how you can limit file uploads to be of a particular type.