I received my Nexus 5X and I finally could check what happens on a 64 bit Qualcomm phone when you don't get ACCESS DENIED: the system files crash :-(
Version 2.5.7 of the installer app fixes this crash and works nicely on a Nexus 5X, in contrast to the Google stock rSAP implementation.
Fun fact: the Nexus 5X does not have sap_security_restrictions set in the baseband's NV storage, but it still works...? I guess there is some more research necessary for the 64 bit Qualcomms.