Check for EZ-Flash Omeg
you probably won't find anything specifically saying that it works on the gb micro because, similarly, they won't say it works specifically on gba sp. there's no need to.
for me, just the fact that the everdrive sticks out turned me off of it.
i would have thought that because it is more expensive, it should be able to fit in a normal shell...
if you cannot live with the fact you need to wait a few seconds after saving before turning the gba off - if that is unacceptable to you, get the everdrive. because in that respect it's perfect.
and i almost forgot, the ezflash omega is open source i think, so that is cool because technically anyone can work on it.