theres probably multiple ways to tackle this but I'll take a stab. You could add an HTML Embed to the canvas and bind an expression to conditionally show it. It will show if the system.origin is equal to your project subdomain. This should work if you don't use the subdomain as your actual site. System.origin translates to whatever the domain you are currently viewing. So on a custom domain it would be yourcustomdomain.com but in the editor it uses the subdomain
I want to display the grid for the design phase and not set the element to display none every time before publishing. Timothy Ricks also has such an element in the lumos styleguide.