I'd suggest a "U" bend at the foot of the fuel tank vent pipe, like on a sink, to trap the vapour in the tank. Of course the end of any vent needs to be pointing downward to prevent any dirt from getting in, so a top bend is also required.
Once the bottom of the U bend is primed, water does not sound ideal, but petrol may evaporate away, the vapours will be trapped in the tank. As the tank level lowers under normal driving conditions, a small vacuum will be formed air air bubbles should slowly creep in. On a hot day when the fuel and vapours expand, the small pressure should be enough to bubble out through the U bend. So long as the height difference between the 2 Us is small, the pressure required to vent will also be small.
HOWEVER, I have no idea if this idea contravenes any manditory venting requirements and I accept no liability whatso ever if you were to try this "idea".
sorry about the crudeness of this diagram.
OR, just put a loop into the existing vent pipe and prime it with a little oil???