To make the hack, first I determined how the gender selection feature works. I used a cheat finder to find out what byte controls it.
One byte in RAM gets changed. Luckily, when you change that byte, the title menu immediately updates the sprite.
Other fun things: You can also poke that byte during gameplay, and whenever there's a screen transition, it selects boys or girls at that time. If you set the byte to something other than 0 or 1, you get garbage graphics.
To make the hack, I found where the game was reading the controller byte for the Title Screen, then added one more comparison. Usually when patching code, you replace code with a jump to some empty space, copy the write new ASM code there (along with the old code that got replaced with a jump), then jump back.