The spray tanker method isn't actually used very much (it may even be out of service now, certainly it was due to be retired at some point)
For something like a pitot, it would be more common to demonstrate AI performance in an icing tunnel, since you can go full scale and totally control the test conditions. Then you obviously also have the probes on the a/c when you do the airframe tests (in "natural icing") and that is used to confirm the tunnel data are valid. (No strange effects due to the presence of the airframe, for example)
The conditions are indeed outside the FAR 25 App C conditions. However, there are new specified conditions in the works, and aircraft moving through the cert process today can expect to have to meet newer, more challenging, rules for pitots etc. Initially it'll be by Issue paper or Cert memo or whatever the authority in question calls an ad-hoc rule (this is where the state of the art is today), and eventually it'll get into the basic regs and guidance.