I want to be wrong about this, but I think there's not a lot (maybe nothing?) that does this. When I looking into BT bridges the problem I found is that one of the ends of the bridge needs to be "smart" (meaning have some kind of UI) in order to pair with the "dumb" (no UI beyond a pairing button) end of the connection.
So if you want to have 3.5mm input -> BT transmitter -> BT headphones, then the thing that converts the 3.5mm audio to a BT signal needs to pair with the headphones and can't unless it's a "smart" device.
@philipmorgan you have the chain right, but pairing isn't as difficult as you are thinking. Stageone's suggestion is the kind of device I am looking for but I am a cheapass and since I need to get a few for family I want to see if anyone has any experiences with cheapies.
I ended up with a Taotronics cheapie for $20 due to an Amazon lightning deal. After some fiddling about pairing (make sure all BT devices nearby are off), it works! And it doesn't sound horrible. However, I have to max out volume to get it listenable. Last time I flew, the volume was crazy loud, so maybe it won't be an issue? I fly again early April so will test and report back.
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