I finally found a code which allows you to play the Secret Track. This track is difficult to unlock normally because it requires 2 GBCs and a copy of both this game and Mickey's Racing Adventure. You are intended to use the infrared link function of the GBC to send a letter from MRA to MSUSA, which should unlock the track.

The code is 01018CCE. Once entered and enabled, simply enter any course in Time Trials (the course will appear glitchy when you load in), then pause and select Restart. You'll be taken to Secret Track.

Be careful when scrolling through the menus, as this code causes them to glitch out heavily. If you scroll too far in the track select menu, the game will eventually crash.

As this track is otherwise impossible to unlock on emulator (and highly onerous to unlock on real hardware), this code is fully legal for speedrunning. Additional codes for other levels can be found here: https:/​/​tcrf.​net/​Notes:Mickey​%27s_Speedway_USA_(Game_Boy_Color)

Track and game crash demonstration: