As far as I know, no one except Shimano makes SPD-SL compatible pedals, and none of the pedals in the Shimano line have flats. A contraption like that simply wouldn't make any sense. There's a well-defined spectrum of target consumers: commuter/occasional rider (sneakers) -> regular rider who likes to be able to walk in his road shoes, or a MTB rider (SPD) -> serious road rider who's willing to accept not being able to walk in his road shoes for the sake of a couple extra % of power transfer (SPD-SL). What you're asking is a pedal that targets group 1 and group 3 at the same time.