There have been some threads about the important plugin Encrypt Tiddler, e.g. this one: Anyone use the Encrypt Tiddler Plugin?
but I don’t think anyone has asked this exact question. Which I happen to have already answered for myself sometime ago 
I have created some tweaks that make this easy, basing on Encrypt Tiddler plugin, take a look here: wilk-tweaks — TW tweaks, settings, styles, palettes.
My additions make it easy to encrypt/decrypt all tiddlers with a given tag all at once using the same password 1. They also prevent from saving by clicking the save button if the tiddlers are not encrypted (it does not prevent saving by keyboard shortcut though).
So this is not a good solution for very sensitive data in a public wiki, but it should be just alright for hiding a part of an RPG wiki.
Feel free to ask if you have any questions about it.
1 As far as I understand, encrypting multiple tiddlers with the same password shouldn’t be a security loophole (as in making it easier to brute force or otherwise break), I’d be happy if anyone with knowledge on the topic could confirm.