Last SP Flash tool version 5 1636 (i’ve downloaded it from need rom) now supports auth file loading.
[EDIT] Current spflashtool home page with download links: http://spflashtools.com/
This is half way to flash bricked meizus, which have a locked bootloaded and so an authorization .auth file is needed to recover them.
Now, we just need the proper auth files …
For a Meizu m3 mini (mtk 6750 chipset), it should be TEST_AUTH_MTK_6750.auth …(not sure)