I actually believe that the adapter to use Lightning headphones on 3.5mm sources will be even simpler and cheaper than the the reverse. Especially since this would help promote upgrading to Lightning headphones. Apple currently has no analogue Lightning devices, intended to be connected to analogue sources. The exception to that would be Lightning headphones and speakers that would need to connect to older Mac and iOS equipment.
Now we know that Lightning is dynamically assignable, so my theory here is that the 3.5mm (male) to Lightning (female) adapter would simply consist of wires and a an identification chip that tells the headphones they are connected to an analogue source. In which case, the Lightning chip will bypass the DAC and route the incoming analogue signal directly to the headphones.
It makes so much more sense than building an adapter that converts an analogue signal (that may have already been converted from a digital source), to digital to get into the headphones, only to have the headphones convert the signal back into analogue. There are no specs providing for this at the moment, but then again, there are no devices that would need this functionality, since everything plugs into the iPhone which is itself only supplying digital signals.