Well well well, things are looking up.
As some of you know Chainfire has created an AROMA utility to configure SuperSU before flashing. My tests are as follows.
Flashed latest Validus 7.1.2 ROM on my G3. Kernel is permissive by default.
Flashed AROMA tool to set it up for system mode. Left all other options at defaults.
Flashed Chainfire's SU 2.82-SR3 which installed in system mode.
Much to my surprise the ROM booted up just fine. Great.
Flashed SYSTEMMODE SU zip from this thread.
Once again ROM booted up fine.
Initially Settings was blank. Tried a few times (close, clear from recents, open again). Just rebooting cleared that up.
Will test again on a ROM with Enforcing by default. If it doesn't work I will use the AROMA utility to force Permissive and see what happens. Most cases though it's usually secpolicy that determines if it'll work or not.
ZTE Axon 7 A2017U, Dark ROM, Tapatalk 4.9.3