This is a fairly convoluted affair. It turns out that any Arduino style board based on the Atmel 34u4 (what is on a Leo) MCU has USB and the extra serial port. I used a Micro as it cheap and all the capability for a joystick.
The joystick library does its magic by creating a real HID device out of the Arduino running it. It turns out there is no way to send data TO an HID joystick.
GameControlPlus is a super library for Processing that provides very configurable access to joysticks and similar. Def look into it - but it IS for Processing only.
In my case the original code I had sent the joystick button press to Processing with a 250ms delay. The response was too quick so there could be clicks received that weren’t timed right. By using the long click, Processing only sees valid clicks. It starts ;a timer which takes the bogie off the radar after it is “shot” - but I have to start another timer on the relay on the joystick board. This signals an Ackman media player to play a loud BOOM sound.
Visit the battleship New Jersey museum ship in a few months, and you can see it in operation. 