Are there any cleats (for SPD-SL) that convert the pedal to a regular flat platform for regular shoes?
For a ready made part, see Fly Pedals.

This is essentially just a metal platform, which you bolt cleats onto, then clip them into your pedals. They are threaded with holes for 2-bolt or 3-bolt cleats, so should work with most mountain bike or road bike clipless pedals (including SPD-SL).

Note these are not yet available, but you can pre-order them from the Fly Pedals website. Some more details on Fly Pedals on Kickstarter - that page says delivery from end of July 2014.