Now, here is my solution. I can create a private key using 20 words. And I can make the last 5 words something only I know. I can email my family (and myself) the first 15 words. Then, I can email them the clues of my last 5 words. Of course, I have to do a damn good job that only the ones I fully trust know the answers. So, I guess my method is 3/4 paper - 1/4 brain wallet?
How do you intend on creating the first 15 words? If it's by using the old brainwallet concept:
An early old-style brainwallet was created by by memorization of a passphrase and converting it a private key with a hashing or key derivation algorithm (example: SHA256). That private key is then used to compute a Bitcoin address.
Then the source also points out that humans are not a good source of entropy, and nearly everything you can come up with is likely insecure.
If you plan on generating the first seed words by something like Electrum (12 seeds, derived from the master private key, not the other way around), and extend it with extra words, it
theoretically should be fine. Since you can add just about any character or word to the pool of seeds, it would probably be nearly impossible to brute force it. I would still be
very wary about keeping partial copies online though.