But if a manufacturer wanted to only play audio to their device, they could have already made it a lightning-connected device.
If they do give the ability to distinguish the lightning-to-3.5mm adapter from other lightning headphones, that isn't any greater than the ability to distinguish between the 3.5mm jack and the lightning port is it?
The issue not likely to arise with hardware manufacturers. However, content license holders have been eager to have powers to restrict playback based on the device that will be playing back, see region encoded disks and HDMI.
If they do give the ability to distinguish the lightning-to-3.5mm adapter from other lightning headphones, that isn't any greater than the ability to distinguish between the 3.5mm jack and the lightning port is it?