How much pressure from gravity system? Dump Zone.

My manual says to hook up my dump zone to the largest upstairs zone.

My largest upstairs zone does the following:
1. From boiler up into the living room then back downstairs.
2. Up into a bedroom then back downstairs.
3. Up into the bathroom then back downstairs.
4. Up into the dining room then back downstairs and then it returns.

My question is upon power failure is the pressure from the heated water alone capable of pushing water through all of these ups and downs? My only other option would be to run it to the smallest zone upstairs which goes up through approximately 12' of baseboard in my den and then comes right back down to the boiler with no twists and turns.

I am leaning toward the latter of the two but am not sure if this will be enough dump for a 90 btu boiler.