Reading Chapter 11 (Power Supply) I don't really get a feeling whether it should be possible to use a GoldCap (say 1 Farad) at the Battery pin - just to power the RTC for a few hours when VDD5V is not available.
It should be possible when using the linear regulators or the DC/DC using the VDD4P2 path.
The GoldCap itself has roughly 100 Ohms of internal resistance, so any switchover event resulting in a larger current from the Battery pin might bring the voltage low enough to trigger a problem.
Is it possible that such a brief current surge can happen on the Battery pin, for example when VDD5V is suddenly lost?
Considering the rather large current on the Battery pin during off state (13.5 typ/30 max) uA, a GoldCap solution might not be the best anyway, but in our typical applications a loss of VDD5V is not the operational case; or if, it is usually brief; or if enduring, a loss of time is acceptable.