Another ecorenovator has considered this approach also. He purchased a mini split unit to replace the electric resistance heaters he used to heat with and did a self install. He was planning on doing a borehole since he has a shallow water table. He was going to run the well water through a car radiator on the suction side of the outdoor unit. Never heard anything about it.
EDIT
His idea is here:
http://ecorenovator.org/forum/geothe...efficient.html
http://ecorenovator.org/forum/geothe...ini-split.html