I want to do the same thing or similar. Something everyone is missing here. If you are using Shared Hosting and one Web site is on server A and another site is on server B, using the dll solution you would have to copy the dll to both servers anyway. Unless you are saying that you can reference the dll with a remote path (http://), which you cannot do with the Register tag.
I am also writing an application that would need a shared ascx file. I want to create the control once, and use it on all my sites from the same location.
Only other solution I can see is an automatic updater program.