What are your thoughts on using a JetCat or similar small turbine for a backup Engine? It could be used to increase takeoff/climb performance. These small turbines are lightweight and come in thrust levels that could sustain level flight in many light singles. Although they are fuel hogs you could give yourself some propulsion redundancy (extended gliding range?) without adding too much weight. Something like 1L per min of consumption, 15 min would be roughly 25lbs of fuel. It could be worked out to see what a good compromise of fuel load vs time needed to get to the airport for a specific make. The exact implementation would be airframe-specific (thrust lines and all) but these turbines have been used as the primary propulsion on several aircraft already. Perhaps a retract system could be used to eliminate any drag penalty like in this glider.