Ah, sorry, no I actually meant the home directory in the case of postmarketos, although I wouldn't mind extending the concept to Android. It seems like you can get 90% of the benefit with 10% of the difficulty if you have an unencrypted OS but mount your home (well "user data") from an encrypted filesystem. Heck, I can't think of any reason that on a normal unix-like[0] you couldn't mount your whole home directory with encfs or something after login. This doesn't protect you from evil maid attacks, but for common phone threat models (theft, mostly) this is still a win.
[0] I was going to say GNU/Linux system, but pmos is based on Alpine which isn't really GNU so much as Busybox/Linux...
[0] I was going to say GNU/Linux system, but pmos is based on Alpine which isn't really GNU so much as Busybox/Linux...