[Speedslug]

Well, as propane is heavier than air I don't think there will be any problem with this set up.

I assume the "air bricks" ( I had never heard that term before ) is for allowing the back of the bricks to stay ventilated and allowed to dry out and that they are not for bringing fresh air into the house.

But even if it is, you would have to have a sizable leak at the tank and then it would have to go up hill to enter those holes.
Not likely, unless you let the vegetation get halfway up the cage in height.
Even then the same vegetation will grow in front of the lower air brick and reduce the chance of propane entering there.

I think you are good but the real question is what will your insurance company say if you have a claim for ANYTHING else that happens at your home.

They can deny a claim for hail damage on the roof if you keep more than a pound of propane inside the house even though one has nothing to do with the other.
They will call it a violation of the terms of the contract and deny everything they can.
They are in the business of taking in as much money as they can and then paying as few claims as they can after all.