Can I open one template/route when Craft is offline? I've set Craft offline (System status). Is it possible to "open" one specific URL route so it can be accessed even though the website is offline?

For example the route: /blog/post
You can add a template called 503.html to your /templates folder. Craft will then use that template whenever the system is turned off, due to the HTTP status being set to 503 (service unavailable).

Inside the 503.html template, you can of course include another template (e.g. blog/post).