>would a ferrite loop work at all inside a 22 gauge steel box,
Not well at all, no.
>is there a standard port I could add to the cabinet to suport an external removable AM antenna,
Use whatever connector you have lying around that's easy to work with. It doesn't matter very much. 1/8" audio jacks would be good. You could probably repurpose an antenna for a Countycomm GP-5.
Or maybe something like this:
https://www.amazon.com/Eton-NGAN200-Grundig-Indoor-Antenna/dp/B001PNNXGO
https://www.amazon.com/Eton-NGAN200-Grundig-Indoor-Antenna/dp/B001PNNXGO For low power, these RX mag loops work nicely for transmitting. I use one when demonstrating radio to young'ins at the museum.