I think you can use the same mounting brackets as for a wall. The piece that mounts to the wall will need to be attached to the iron fence using steel brackets and just bolt it together to remain on the fence. Then the mounting piece that goes on the TV would stay attached to the TV and just latch onto the fence bracket as needed. Then just release it and pull the TV and its mount off for storage. Obviously you have to run a plug and other connections to the TV but that all can just be plugins and stored in a weatherproof box by the fence.
My thoughts are that you have a metal straight piece that is on back side of fence you can use small u-bolts to hold it firm and at desired height. Then the TV wall bracket (that mounts traditionally to the wall) is now mounted to the metal bracket using bolts and nuts. Then the TV bracket (that screws to the TV) is unchanged and simply hangs on the wall bracket as designed.