Changes

Dolphin devs are working at rewriting the entire GBA connectivity code in a far better way from scratch with more accurate emulators. They did a video using the higan emulator. Nothing of the sort is published at the moment.

Inserting a GBA card in Slot-2 in a Nintendo DS unit (that's not a DSi) while a DS game is running could unlock various gameplay features in some DS games. DesMume can emulate this: while playing the DS ROM, go to "Config, Slot 2 (GBA Slot)" and select "GBA Cartridge". Now select the GBA ROM file, and make sure its sav file is in the same folder. You may need to reset the game sometimes to see the effect in-game.