How can I best fake an (AM radio) antenna signal?
If you want to feed sound to an AM radio, you don't necessarily need to make any direct connection to it at all. You just need to have a coil nearby that magnetically couples a suitable AM signal into its antenna.

There are many examples of low-power analog AM transmitters on the web. One way to generate a very high-quality AM signal using modern digital technology would be to get one of the many inexpensive DDS (direct digital synthesis) modules (or just a bare chip) and feed digital audio from any source into its amplitude control register at a suitable sample rate.