zugswang

It's not there because the game is classed as Not Working in MAME. One it is elevated to working in the future it will show up.

They are 4k. I am obviously no expert so I will leave it to Launchergirl to do it for you.

I can confirm that this works fine via bat files. Like you say it edits the dolphin.ini and changes the GBAPlayerRom= line to whatever game you want loading, then loads the Game Boy player fullscreen.

The Genesis Plus GX core allows you to remove the side borders on Sega Master System games. How to Remove the Left Border Launch a Sega Master System game in RetroArch. Open the Quick Menu (press F1 on a keyboard or your assigned hotkey combination). Go to Core Options (or Options). Look for the setting named Borders or Hide Master System Side Borders. Change the option to Left Border Only or disable borders entirely

Super Mario Land.