Does DNN contain functionality to put a 'Down for Maintenance Page' as a default when making changes in the website?
It must be a custom-made page, not a default DNN-page.

Off course I can create this myself, but I'm hoping there's a quick way to put my website 'offline' when I have to make changes for a client.

(I'm working with the Content Staging allready, but changes that does not apply for changes in f.i. the skin)

If there is a specific function for this, I've never come across it. What I do is simply make pages visible to Admins only whilst creating/editing them and then make them visible to All Users when ready. I suppose you could also create a splash page informing people that changes are being made (set this in Admin > Site Settings | Advanced Settings).

I obviously don't worry about my visitors as much as you do! As I said, I've never come across a function to take a site offline, that's not to say there isn't one. Thinking about it, though, if there was, you could find yourself locked out as well. Hopefully, someone else will comment.