Egress lighting is outside of the scope of the fire alarm, I’d take a different approach.
I’d recommend putting a normal flood light connected to the 120vac near the ladder and have it always on. As a backup, put emergency lighting on the same circuit as the 120vac flood light. When the emergency light loses the 120vac connection, it’ll turn on illuminating the ladder. Downside here is it’ll only last a few hours probably.