I've done this before with the NES and SNES versions so why not GBA? As always, fake doors just spit you back out.
This video was recorded with BizHawk. I opened the hex editor and made the following changes in the EWRAM to get my mushroom block.
01AE = 14
01AF = 02
EDIT January 2022: It occurs to me that I should have translated my hex notes into a Gameshark code for people who maybe want to do this on the actual hardware. Well here it is.
820001AE 0214 Place A Mushroom Block In Front Of The Intro Door
![](https://i.ytimg.com/vi/-1HEUp8AXq8/mqdefault.jpg)