Keep in mind, the RTC battery substitute module allows the main laptop battery to power the RTC circuit instead of the RTC battery, so if you use the substitute module and no main laptop battery, the RTC circuit will power down any time you remove power from the mainboard, even for a moment.
Someone else will have to clarify if the BIOS re-sets any time the RTC circuit is no longer powered. But I noticed you said the mainboard will never go without power for more than 1-2 hours in most cases. Just be aware, that the battery substitute module doesn’t power the circuit at all if there is no laptop battery connected.