I know the 396/496 will push to an SL-30 or SL-40. I have a 396/SL-40 combo in mine and it works great. Just 1 wire (the serial out from the 396 to the RS232 in on the SL-40).
It seems a few radios have the ability to be controlled remotely, I wonder if that is the driving factor. I like the look and panel size of the Becker radios, which advertise they can be remotely controlled with a serial interface, I wonder if that means a garmin portable can freq push to this unit.