Hi Patrick,
Your first link is going to your custom 404 as well, but I get what you are describing, so yes, if it were me I would make sure that the pages are noindexed (using the noindex meta tag, which is the most reliable method) by default when they are created. When you decide that a page is to remain on the site permanently you can then remove the noindex tag and make it visible.
Your custom 404 has everything you need to make sure there is a good chance the visitor will click through to another page in your site, so you are doing all you can to make sure that those people following a broken link don't bounce right back to the search engine.
I would say your logic is correct in that the only way to further improve the odds is to reduce the potential for broken links before they happen.
Hope that helps,
Sha

