The schematic was released in german magazine "Schneider Magazin, 07/1988", apparently using 5 diodes and a 4.5V battery.
Aside from Atari-ST mice, the circuit should also work for Amiga mice, which are basically the same, but with XA/XB/YA/YB connected to other pins, which could be resolved at software side (or at hardware side).
CPC Joy Port Atari-ST Mouse Amiga Mouse JoyUp Pin1 --|>|------------- Pin1 XB Pin1 YA JoyDown Pin2 --|>|------------- Pin2 XA Pin2 XA JoyLeft Pin3 --|>|------------- Pin3 YA Pin3 YB JoyRight Pin4 --|>|------------- Pin4 YB Pin4 XB JoyFire2 Pin6 ------------------ Pin6 Left Button Pin6 Left Button JoyFire1 Pin7 ------------------ Pin9 Right Button Pin9 Right Button JoyFire3 Pin5 --|>|------------- Pin8 GND Pin8 GND JoySel1 Pin8 ------------------ Pin8 GND Pin8 GND Batt4.5V (-) ------------------ Pin8 GND Pin8 GND Batt4.5V (+) ------------------ Pin7 +5V Pin7 +5V
The low-level XA/XB/YA/YB signal must be watched by software (the 300Hz interrupt is probably too slow to sense fast motions, so there's little chance to have the mouse driver running in background).
The circuit reportedly doesn't work on CPC Plus computers. The Atari-ST adapter is supported by G-Paint 2.0.