Powering up a panel using only battery backup isn't a Code requirement. The National Fire Protection Association Code (NFPA Code) is a guideline showing the minimum needed to properly protect from fires.
The concern for the NFPA isn't about how the fire alarm system works while it is being assembled; the concern for the NFPA is about a completed fire alarm system in an occupied building. Powering up a panel using only backup batteries would only be done while testing, servicing, or building a fire alarm system, so the NFPA doesn't address this kind of issue.
Powering up a panel using only battery backup is a manufacture's design issue. Some manufacturers have "fail-safe" features that prevent a panel from turning on until utility power is applied; some manufacturers don't bother with the fail-safe features and allow the panel to be powered up using the batteries.
Some manufacturers design their equipment both ways. They protect their control panels with the fail-safe features while not bothering with the fail-safe features on their power supplies.
If the building isn't complete enough to get utility power to the panel as you're working on it, you'll have to get utility power to the panel some other way. I have been known to make a temporary cord and plug so I could work on a panel before the utility power is set up by the electricians.
Talk to the electricians, perhaps they can help you get temporary power.