Programming the tankless to output ~150F and putting a tempering valve on the tankless output should be able to protect any units that can actually be damaged with low delta-Ts on the heat exchangers. When the incoming temps are high the hot water flow through the heater will often be lower than the min required for ignition (typically 0.5-1gpm) for most draws, and higher flow draws can take the heat of the burner activity. But this will also lead to short-cycling & temperature fluctuation issues at low flow.
The Rinnai-authorized solution will work better at low flows and have better output temp regulation overall. What don't you like about it? And what sort of "damage" are they talking about? (If it's wear & tear from short-cycles, there are no low-delta-T solutions that will avoid that.)
Another solution (and perhaps the right one) is to install a small buffer tank (8-10 gallons- a small well-insulated electric HW tank would do) plumbed as flow-through from the geothermal, but with a circulation pump on the tankless using the tank's aquastat to turn on the pump on the tankless loop. Program the output of the tankless to at least 20F above the setpoint, and with sufficient pump flow that when the tank temp is 15F below the setpoint the tankless modulates up to at least 50% of full output. That way, the tankless just maintains the temp of the tank with big delta-Ts under all conditions, and can be set up to NEVER short-cycle (since the tankless output temp, pump flow, hysteresis of of the aquastat, and the volume of the tank determine a minimum burn length- this may be tweaked using multi-speed pumps and playing with the output temp of the tankless.)
When the flow from the geothermal is over the setpoint the tank the tankless never fires- it's slaved only to the setpoint of the tank, not the hot-water-use flows. This fixes forever the cold-water-sandwich and low-flow flameout issues as well. Even though some tankless manufacturers void the warranty on tank-temp-maintenance applications, properly set up it actually SAVES the tankless and increases the operating efficiency (for a very small penalty in tank standby loss) by reducting the total number of ignition/burn/flue-purge cycles. This also increases the peak flow potential, since the hot water runs through the big-bore low-head plumbing of the tank, not the high-head of a tankless heat exchanger.
Let the pump handle the head of the heat exchanger. Setting the tankless outputs for a high delta-T the pump flow doesn't have to be all that high either- 4gpm @ 50F rise is ~100KBTU/h. If you set the tank to 125F and the output of the tankless to 175F and the pump flow to 4gpm a tank temp recovery burn is a healthy~100KBTU/hour out by the time the tank has chilled to 110F from high-rate hot water flows the output of the tankless is roaring along at 130KBTU/h out. (Most 199KBTU tankless outputs max out at ~155-158KBTU at full fire.) You can tweak the flows and output temps to find what works, but try to set it up with the recovery/maintenance burns long as possible to maximize low-flow/low-use efficiency. Lower fire maintenance burns will increase system efficiency, but will lower the effective maximum output of the tankless- there's a balance to be struck as a matter of practicality. But unless you have multiple showers going on simultaneously, the buffering of the tank means you generally don't NEED max output- the mass of the tank masks the shorter draws that occur during the longer draws from the tankless.
With a 10 gallon tank, 100KBTU/h recovery burn and a 10F hysteresis on the aquastat you're looking at about a 30 second minimum burn- perhaps longer than a hand-washing burn on a normal tankless setup but still less than ideal. A minute or more would mean the tankless always ran near it's raw combustion efficiency. There would still be far fewer burns though- the modulation during the minimum burn would alway be much higher than a low-flow hand-washing draw, and many short draws would occur between burns. This is really no different than a boiler & indirect-tank except that the "boiler" in this instance is a tankless, and instead of a heat exchanger and a boiler-water loop, the tankless is heating potable water directly.