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Not sure this would work out of the box on R8x either. 

The issue is, to achieve NAT Hide on a non-GW IP, you need to use a Dynamic object for the active ISP NAT-Hide address in the Nat rule, and that IP should be changed when failover happens. Even there, there might be an issue with a n automatic ARP after failover. 

We have a SecureKnowledge article describing ranges for ISPr and scripts to switch those in case of failover. It is sk174197. 

This might work for you, if you replace a range with a single non-GW IP address (effectively /32 range).

Concerning the management decision, having ability to open a support case should be enough reason for me. Do you even have a support contract in place?