Flashback guaranteed restore point with ADG for DB upgrade fallback

I've been working on a plan to get our databases upgraded from 11.2.0.3 to 12.1.0.2. Most of the planning is relatively straightforward, except for one DB. It has active data guard to a physical standby. I've managed to work out how to properly upgrade it, despite the "instructions" at
https://docs.oracle.com/database/121/SBYDB/upgrades.htm#SBYDB4931

The part I can't figure out is how to rollback the upgrade in a timely manner, should it be needed. The easiest is to restore from backup and recreate the standby, but that will take several hours. https://mikedietrichde.com/2017/08/29/fallback-strategy-flashback-to-guaranteed-restore-points/ says to use Flashback guaranteed restore points, but there's no mention of how this should work with a standby. No problem, I can figure that out. The restore point on the primary doesn't exist on the standby, but I can get the SCN, subtract 2 (as per MOS articles), then FLASHBACK STANDBY DATABASE TO SCN nnnn. That produces "ORA-38726: Flashback database logging is not on.", since, well, flashback isn't on. It's not needed for the primary -- is that a restriction on the standby?

Anyone know 1) where this might be documented, and 2) how one might go about planning a time-sensitive upgrade fallback for an ADG setup?

I was thinking that instead of relying on the SCN for the Flashback on the standby that maybe I'd need to setup a separate guaranteed restore point (with MRP stopped) before the primary one is created. I've been testing various scenarios, but as it takes >30 minutes to set up each scenario, I thought I should ask here to possibly save me a couple of days of fun.

Thanks!
Rich

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