DokuWiki does not use a database. The contents of all pages are kept as simple text files in the file system. This is not particularly safe for storing highly sensitive data, unless you are good at hardening your server.
Even if you managed to keep the contents of that part of the file system safe, I would not rely on any plugins to hide crucial information from the members of the wiki. Hiding parts of a page from unauthorized users is not part of DokuWiki's architecture. I would greatly fear that other plugins might unknowingly undo the safety of Myron's plugin.
You could avoid that part of the problem by placing all sensitive information on pages that are completely invisible to unathorized users. You then could show parts of those pages selectively in more public pages by using the include plugin. This would, however, still leave the risk of anyone accessing your files on the server.
I use dozens of accounts on many servers, both local and in the web. I use but one tool for keeping all those credentials, a password manager. To be quite secure, I use a password manager which is not integrated into the OS or even the browser. I have decided on KeePassX, which is available on all platforms I use, and I use the means of the various OSs to synchronize the password file across all environments where I might have need of the credentials, and this includes the phones.