A GBA program that uses the link port to communicate with a HC-05 bluetooth module using UART (HC-05 flashed with RN-42 firmware, find how to do it in the links at the bottom).
This rom is stored in a W25Q32 flash chip. An Atmega328P performs a multiboot sequence reading from this chip and sending to the GBA through the link port.
Once multiboot ends, the gba program runs and talks to the HC-05 to handle bluetooth connection, and key presses.
Additional 74XX157 quad 2-input multiplexer makes possible to multiplex SPI to read from the flash chip and send the multiboot, it alternates byte reads from the flash chip and send them to the GBA. Then using hcf4066 switch link port pins are switched from the spi to the HC-05 uart pins.
The whole circuit runs at 3.3V provided by GBA through the link port.
Final version of the circuit includes a 6 pin port that allows to reprogram the atmega if necessary. Also it lets to turn the atmega into a special write mode to reprogram the flash memory.