Someone asked me if we can connect HT-1A to QRP Labs 50W PA. I have consuled with the designer and he said we can use MUTE line (pin 3) instead of KEY line (pin 4) in J1 (the connector to connect the main board and the display board). The MUTE line has a bit switch delay and the voltage is a perfect match to the PTT input of the QRP Labs 50W PA. You can drill a hole to the rear panel of the HT-1A and install a jack for the PTT output.
If you have both of them, you can give it a try and let me know the result. I believe it should work perfectly.