* facepalm *
The official adapter is compatible only with GameCube controllers, from its USB interface you can't get anything else besides the status of buttons/axis. That's it, there's no way that a custom driver or anything like that could make GBA-Link cable work because the adapter doesn't provide the required communication interface. Got it? A possibility would be writing a new firmware for the adapter, from scratch, involving reverse engineering and things like that, very unlikely to happen. Also, if Nintendo used ROM to store the current firmware or even worse, if the firmware is built in the main IC from the adapter, uploading a custom firmware to the adapter would be almost impossible too...
The official adapter is compatible only with GameCube controllers, from its USB interface you can't get anything else besides the status of buttons/axis. That's it, there's no way that a custom driver or anything like that could make GBA-Link cable work because the adapter doesn't provide the required communication interface. Got it? A possibility would be writing a new firmware for the adapter, from scratch, involving reverse engineering and things like that, very unlikely to happen. Also, if Nintendo used ROM to store the current firmware or even worse, if the firmware is built in the main IC from the adapter, uploading a custom firmware to the adapter would be almost impossible too...
Avell A70 MOB: Core i7-11800H, GeForce RTX 3060, 16 GB DDR4-3200, Windows 11 (Insider Preview)
ASRock Z97M OC Formula: Pentium G3258, GeForce GT 440, 16 GB DDR3-1600, Windows 10 (22H2)
ASRock Z97M OC Formula: Pentium G3258, GeForce GT 440, 16 GB DDR3-1600, Windows 10 (22H2)