Using Gnome-Disks you can save the de-crypt static password, however after reboot when opening the sdcard folder using nautilus it asks to enter root/admin password. Since the phone is already unlocked it should give you an option to save the unlock root/admin password in the keychain so on selection of the drive it will simply mount/decrypt/open.

It would be nice if sdcard encryption could be enabled and integrated into furios. I think that is similar to what the L5 was doing. I did not have to enter any passwords.

Additionally these root/admin password prompts could also be piped into the fingerprint reader (after initial unlock) all prompts could be answered by the fingerprint reader.