It could be done, but as has been said, availability of NG throws the whole question of doing geo in the first place into economic doubt, unless cooling season is long and hot.

Yes...in a way! Electric backup as you know is very expensive to operate. I used it for one year on my Bard 2.5 ton unit to supplement during the coldest days of the winter. However, the following year I had an idea...why not supplement with a hydronic heat exchanger. I run the heat exchanger off my 40 gallon natural gas hot water heater. I have good control with the unit even in a power failure (since natural gas is still available) using a small portable generator and my GHP in emergency heat mode. You can put one in for little money with some copper piping, small hydronic pump, a couple of check valves, air purge valve, and of course an heat exchanger. The job is simplified if your hot water heater is near your GHP.

Well done.

Issues to beware of would be the water coil's introduction of an excessive restriction to airflow as well as the relatively low btuh available from a typical gas fired water heater. If aux or emergency heat is required for any apprecialble time, hot showers are out of the question for the duration.

That said, if winter storm knocks power out for a few days such an arrangement would allow you to continue to occupy the house and avoid frozen pipes; advantages your neighbors would be sorely lacking.

How are stagnant water problems avoided?

Good point.

Arrangement would require a drain valve to purge stagnant water before opening return water to tank, and since we have no way of ensuring HO will follow purge SOP, so not ready for contractor prime time.

I can envision a modification to a standard storage water heater where this could be made indirect, increasing the safety.

Stagnant water can be eliminated by drawing off a small quanity of water (i.e. low flow) from the system by hooking up a humidifier. This way a nearly constant low flow of water is drawn off to the humidifier pad (and excess discarded to drain). Also, one could install the heat exchanger with a couple of unions (and shut-off valves) so it may be removed at any time (i.e. spring to fall).

Agreed, but any system installed by a licensed contractor has to be foolproof as to user safety - need to guarantee rather than merely provide for safety of potable water