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
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.