I think ChargeFox have got batteries at at least a few of their ultrarapid charger sitesI don't understand why if demand charges are the issue and as expensive as suggested, why aren't batteries being installed onsite? They should more than pay for themselves.
A quick google tells me a PowerPack costs $170k (probably USD) however the Evie guys told me a 2 stall ultra-rapid installation can exceed $1M, so adding a battery doesn't increase the total cost by a huge amount.
A PowerPack can supply 130kW for just less than 2 hours (232kWh storage). If usage really is as infrequent as they say, this is plenty of time to recharge between cars.
130kW would also reduce the grid draw dramatically except for extreme cases (2 Model 3 LRs turn up simultaneously to a 2 stall ultra-rapid and begin drawing 190kW each) and even then it reduces the peak demand by 35%.