i was thinking of a similar mod a while back (but the missus refused ), i was planning on burying one of these 3ft down in the garden....
a domestic radiator....
attaching 2 pond pumps (flow / return) out by the rad, ultimately running a completely silent freezing cold rig..
there were problems with it though, the winter for one... frozen pipes is a real worry (no flow = melting rig)
& getting pups powerful enough for such a large loop while not being too much pressure & struggling to get through the cooling blocks...??
+1 OP...... very innovative
Just keeping the water moving at full speed and having larger ID pipes (for less head loss, and more water volume) would go a very a long way to avoiding freezing over.
I wouldn't mind trying that with an old heater core, a cheap harbor freight Chinese pump, antifreeze + a corrosion additive, and then a plated heat exchanger to keep the dirty loop separate. The trick would be burying it somewhere WET to avoid saturating the ground as much as possible.