True flex-fuel capability requires electronic fuel injection along with a computer control and related sensors sophisticated enough to adjust fuel injector pulses based on the fuel blend in the tank. Given the limited availability of E85 fuel, the car manufacturers had to be prodded by the government to include the additional hardware (there was little financial incentive to do so).
This sort of fuel system might work on a larger outboard motor as well (those usually have a weather-proof engine cover with enough room for the computer, plumbing and wiring). Smaller outboards, power mowers, string trimmers, chainsaws - none of those are suited to flex-fuel configuration.
Sure, you can build a small engine (or a carbureted car engine) to run very well indeed on E85, but it won't run well on anything else. This push for E15 is a way to force increased use of ethanol as fuel because when given a choice, consumers prefer their ethanol to be aged in oak casks and served neat or over ice.
Actually it won't, because the ethanol draws moidture out of the air and with time forms a layer of water/ethanol (which won't run the engine) on the bottom of the tank. I already pay a premium ($5.15/gal * 300gal for a fill-up) for non-ethanol gas at the fuel dock.
I just spent the weekend cleaning the carb jets that were plugged from the ethanol. Boaters beware.