Fired it up today and all is working well.

Thanks. I'll report back with results for anyone else facing a project like this in the future.

I'm tempted to just make up the cable and try it. Can anyone advise on risk of harming either radio by crossing these pins without adding other components to try to match/control input levels?

I've been asked to link two base radios to allow the receive on a XTL1500 to be transmitted on a CM300 (crossbanding one way only). Here's where I'm at with a design using the accessory ports on both radios: XTL1500 Pin 1 Ground to CM300 Pin 7 Ground XTL1500 Pin 21 Receive Filtered Audio Out to CM30...