It was (is) a modified air-air refuelling tanker with a special spray bar attached to the refuelling boom. The smart bit was the spray bar, since it has to 9try to) give a controlled and known set of water droplets - just dumping a large amount of water in the air doesn't replicate an icing cloud.
IIRC the icing tanker could only reproduce certain ranges of droplet size, and maybe only certain sizes within that range on a given flight - I think they had interchangeable spray bars for different droplet sizes.
So any airframe with tanker capability would do, but only if fitted with the spray bar system.