I toyed with the idea of making a simple IR receiver board to simulate what is in the wired connections, but I didn't see the market, so I let it slide. I suspect it would be fairly simple, but for the K-Line, you'd lose the backup light as that comes over the tether. You'd also lose your electrocoupler, another thing that comes through the tether. The AD20 was there to support those functions.
If you really want to do this, the hit ticket is to buy the proper and AD20 from Lionel and start wiring.