I had this idea but never executed because it's only bitter cold outside for about 3 months out of the year. And then I'd basically be doing a semi-annual detachment/attachment of my PC from the wall and probably removing/adding an insert from a window for the other end of the loop.
This combined with my roughly 4 month itch to rearrange my office made it seem like much more of a pain in the ass than it was worth to ever actually do.
I've done it a few times. I find that seasonal temperature changes are not as much of an issue as I first thought. Automotive radiator fluid basically never freezes. And once you have a coolant loop running outside, your options are far greater. Even radiators are a unnecessary. I once has a coolant loop run outside to 20-gallon washtub sitting on the ground under a wood deck. No radiators or fans. The entire rig cost less than 100$. Just a tiny aquarium pump pushing water up the loop into the house. The tub was in thermal contact with the ground and was so massive that it took many hours for my computer to warm it up even in high summer.
This combined with my roughly 4 month itch to rearrange my office made it seem like much more of a pain in the ass than it was worth to ever actually do.