Well, I'm late reporting this, but if anyone is still following this thread, it may be of use to them. Someone sent me a PM many months ago with the solution. I can't find the message, so can't give credit here, but can only say that I didn't figure it out (and neither did Alinco).
Manual 6-9
Transmission Inhibit (PTT Key Lock)
"If "ON" is selected, the transceiver can operate only as a receiver."
Press FUNC key.
Press FUNC key again and hold for longer than 2 seconds to enter SET mode
Press the DIAL LOCK key.
Use UP/DOWN keys to select ON or OFF
Press FUNC key to confirm and exit SET mode.
NOTE: Inhibiting transmission does NOT disable the built-in keyer and oscillator tone, so you CAN use the DX-77 as a practice oscillator.