I think in this case it would be ok if it wasn't reserved per se. If a user used that tag but didn't have the plugin installed, nothing special would happen. If they added the tag on their own with the plugin installed, it would automatically make the note into a template.
This tagging system is how the Persistent Editor Layout plugin works. I can add a layout:viewer tag to any note and it works. If I delete the plugin, my note still has the tag, but the tag no longer does anything fancy.