If an electric preheater is more efficient than a propane-fired preheater, then people who don't go electric for the boil can at least benefit from some sort of immersion type 120 VAC preheater, which would be much cheaper to build than an e-Vaporator and wouldn't require routing a 240 VAC circuit. That's where I see the benefit of adding an electric preheater. It makes the gas burners more efficient. My intent is to stay a small scale hobby producer of maple and birch syrup, with 20 - 30 taps max for maple and 10 or less for birch. So I like the flexibility of being able to use other methods when the primary boiler can't keep up with the sap influx. On-demand point-of-use heaters seem like a useful idea if the temp can be set high enough. Coffee makers and room humidifiers heat water to boiling temp, so they seem like natural candidates for a kluge. I'll look into that after I get my In-Sink-Evaporator running.
The solar preheater could be made with a used Dish or Direct TV antenna with small pieces of mirror or aluminum "furnace" tape glued to the curved surface to focus the sunlight on a glass or metal cup. A Sun-follower to move the antenna isn't that hard to make. Someone else is more than welcome to work up a design for that. I do too much of my boiling at night to think about daytime boiling.