I have neither the skills or the tools to do this, but I did design (in my head, cause there is lots & lots of room ) a water tank to sit on the stone wall & melt snow &/or keep the water from freezing. A metal tank, maybe like a "Jerry Can" with a spigot on the side away from the fire. If a Jerry Can, maybe put the spigot on the end with the lid, this would serve multiple uses: it would put the opening away from the fire so the gasket wouldn't melt & allow for easier filling, and would allow ease of drawing water out.
MY design is more like one of those 8 gallon "coolers" like used at construction sites, only metal. With a large lid for ease of filling, but close-able so ash & smoke won't get in. In either case, I think, the spigot should be a bit longer to allow it to hang away from the wall, & still have the "Can" heated by the fire.
A point in favor of the Jerry can, it is narrow AND wide, depending on how we position it. Narrow will allow more heat past to warm those gathered, but if turned wide side towards the fire, will allow more heat to affect the snow/ water inside. It would also require less work as the "tank" is pre-made, just drill a hole & put in a faucet.
I will try to make one of these in the next few months, but as I said, I do lack the skills & more importantly, the tools. But I do have access to a army surplus store & a Lowes.