We recently performed this "hybrid" analysis for a college in upstate NY (heating dominated climate).  We can reduce the # of wells by 20% by adding only a small boiler that will need to run less than 200 hours per year.  A 35% reduction in wells adds 200 more hours/yr of boiler use.  The extra cost (and space) for the one condensing boiler along with pumps & controls still creates a truly dramatic first cost savings - plus increases reliability and fuel diversity.  The GHG reduction (typical with GSHPs in the NE US) is barely impacted since the net annual heating totals from the GSHPs is still 95% or 90% respectively (this due to so few hours when really cold).  Annual energy costs are effectively identical to 100% GSHP.  In this climate, the well reduction doesn't force a fluid cooler (to supplement summer heat rejection) to be added until the wells are further reduced below 35% (of peak heating need).

From Ted Sumrall:
Closed Loop Heat Exchange

"This is the right track. Now - if we just take it one step further. If we ran a piping system (and it could be very small in the x or y direction, but have a number of coils) in the area of historically active volcanos (where the heat still exists but the volcano is dormant) it could be very functional for power generation. As long as it were closed loop, little water would be required for heat generation."

There are actually some very interesting developments relating to low temperature power production.  United Techologies has a Pure Cycle machine that will produce power with 195F water.  This is geothermal power production in contrast to what we were discussing which is geothermal ground source heating/cooling...different applications.