Is there some particular reason not to put new holes in the bricks? My first inclination is to just drill new holes, and secure the spouts properly - after all, they did come off during a snow storm.
Do you have pictures of the existing mounts? Perhaps there is a way to retrofit something onto the existing hooks but still use a modern downspout.
May I ask you to temper or clarify one adjective? Is this water literally BOILING hot (hotter than the tanks) or only SCALDING hot (as hot as the tanks, about 120*F)? If the tanks are also used for heating, then perhaps 180*F?
If the tanks are used both for heating and domestic hot water, then you have several large tempering valves, correct? These tempering valves mix hot and cold water together to achieve the industry-standard maximum 120*F for domestic hot water.
You're on city water, surely? High pressure, enough to push water to the top of the building, with pressure regulators on each floor?
This is the sort of thing that might happen if a tempering valve fails or is misadjusted, AND one or more of the tanks has developed a large air bubble. It's more likely to happen during periods when the municipal water supply pressure falls (because other nearby buildings are also using a lot of water). The hot-water tanks have been pressurized by high municipal pressure, including the one with the bubble, which makes it into a large surge tank. When municipal pressure falls, pressurized hot water backflows through the failed or misadjusted tempering valve into the cold-water system, where it's delivered to faucets, toilets, etc.
A really good plumber should be able to track down the source, but it'll take some time. They'd have to carefully measure the temperature of the pipes in the pipe chases, following the hottest pipes towards its hottest end until they come to the hottest spot in the cold-water system - that'd be the tempering valve in question. It'd be BEST if they could work while the occupants of the affected units were not in the building, so the water to their units would remain pretty static... making measurement easier.
The only other cause I can think of would be backflow (intentional or accidental) through a bathtub or laundry outlet, where hot and cold could be mixed without actually dispensing water - in the case of a bathtub (which has much larger water connections than a faucet or toilet), it'd require that the spout be blocked while both hot and cold were turned on. This would allow hot to backflow into the cold-water system.
I'm leaning very hard towards the tempering-valve problem, though, because otherwise we're talking about collaboration between numerous occupants in different units or a really odd set of coincidences.
Best Answer
What you're looking for is called a proportional valve and they're typically not cheap. Here's a series that should do what you want from ASCO, and I was able to find it for sale online for US$316 (as of today). I'm not sure if that price includes the PWM controller module or not, or how necessary that module is.