This DX-400 sounds like a receiver I'd like to try and get on the used market or is there another type I could look for that has a transmit relay? 
The FT-102 had as an option a remote VFO which permitted split through the VFO controlling the RX, TX or both in normal mode. It also had a keypad for locking and direct frequency access. 
A great option if I could find one in top condition but those who gottim, aren't parting with them and for that matter, the FT-102s them selves that are in top condition, aren't going anywhere. 
I've only come across 3 FT-102s for sale or trade the last 3 years and I have one of them. Sorry went off course a little, because of the rarity of the remote VFO, I was considering a receiver that I could use instead. 
The DX-400 type might be able to accomplish this task but what about the front end? Could you hook up a receiving antenna to it that could be mounted on a rooftop, say, 50 feet from your transmitting antenna and run at least 200W and not hurt it? What about 600W? 
I'm just wondering why this relay would be even in the DX-400 if it couldn't unless it was for home built 5W QRP rigs. Then there's triggering the relay to mute the receiver to transmit in one complete operation. How would this work? 
Would you have to have a hand or foot switch that you would activate before you'd key your transceiver? Maybe it might be best to look out for a Drake RX4 type, adapt it to the FT-102 and go from there. 
The RX most likely can handle big signal power because of its uses in a twins configuration with amplifiers and I'm sure the receiving antenna was probably close to the transmitting antenna. Unless the twins setup, shared the antenna but wouldn't the front end capability have to be in the Drake RX? 

73: 
Mike VO1AX