Okay, figured it out; It’s simply not possible ??
If anyone experiences the same problem here’s why: Since WordPress internally rewrites all URLs and also does some magic detection stuff there there’s just no chance to let Apache know if the request will result in a 404 or not, thus the only option is to use the WP-internal 404-errorpage.
This is not a WordPress-Flaw, it’s much more a technical restriction one has to deal with and as long as URIs are being dynamically rewritten to map “physically non-existant” URLs only the last one in the chain, the one doing the rewrites, can find out if there is content or not.