With ReDJ you can redirect all URLs that match a certain regexp pattern to a single destination URL.
Unfortunately, you want to do this only if a 404 error happpens, and this is not possible at the moment without modify the code. You should change redj.php in plugins\system folder. For example, you can force to exit without apply redirect if there aren't errors. This can be done adding this row after line 93:
if (JError::isError($object) == false) return;
But in this way you can't do a redirect without an error.
This can be a good new feature to implement in the next release.
Regards,
Luigi