Husse, did you observed total GRUB crashing??
What i could observe with the guys trying it is that some pictures works, and some not, worst case is get a grub with black background, but is mentioned on the top of the other post the fix for it (just changing a symlink), hey, good idea, i'll add this option (fix). EDIT : already added on version 0.3.6 =)
But i'm trying to get in touch with guys that know more about grub-gfxboot because is some image specs that i'm missing, is causing some pictures to work, and some not....
Anyone who wants a more easily customized grub is encouraged to test, and if you do, please, help me to figure out what's wrong with some images =)