Someone forked Warp Wallet to spit out a 12 word seed (as well as the extended private key), but still haven't found one that directly goes to segwit addresses. You could do a multi-step procedure to use a randomly generated password to spit out yprivs and zprivs if you like to have a segwit brainwallet.

I found these:
https://github.com/Logicwax/PortalWallet <= used older Warp Wallet code, and doesn't have segwit.
https://www.nowallet.org/

The only use-case for a pure brainwallet, as others have mentioned, is when escaping a country or government or warzone with only the clothes on your back. Even in this case, you can probably sew on embroidered password under your clothes or otherwise write it down somewhere on your person.