I have heard about a new technology that makes heat pumps more practical for very cold climates and avoid the "cold blow" problem at the same time.
Instead of switching over the heat pump for defrost, it uses a completely separate system for defrost.
In the outdoor unit, the coil has a wire embedded in it. It is insulated from the coil itself. It connects to a high power magnetron on one end and a sensor on the other. In normal heating operation, frost builds up on the coil. When a sensor detects the need to defrost, it stops the fans and compressor and starts the defroster. A high voltage transformer charges up some capacitors and feeds a little power to the magnetron. The small amount of electromagnetic radiation melts a little frost built up on the coil, causing the water to stick to the wire by surface tension. A moment later, a relay discharges the capacitors through the magnetron. The very intense burst of electromagnetic radiation is converted to heat in the water sticking to the wire, causing the water to flash to steam and blowing some frost out of the coil. This repeats several times until a sensor indicates the end of defrost in which case the defroster turns off and the compressor and fans turn back on.

I have not been able to find any more information on this. Has anyone else heard about this? I'm aware of only one unit using this technology, and that's a unit cobbled together by an amateur out of a normal heat pump, some surplus defibrillators, a surplus magnetron, and a few more parts to tie the design together.