Yes, of course, we would intend tags added to submodules to work the same way as tags added to the main repository work currently.
We don’t currently, display “tag change” changesets differently - you can usually tell by the commit message. We will be looking into adding an indicator of some sort to them in due course.
Continua CI would need to know when to query for a change on the submodule. So you would need to set up a “code pushed” web hook on the submodule repository for each main repository in Continua CI which references that submodule.