This has been fixed upstream since commit b8765fa0 (2013-12-17)
https://git.savannah.gnu.org/cgit/grub.git/commit/?id=b8765fa0824aab891a28b728f9b7309cbe5c6ba2
Excerpt from util/grub-install.c:
efidir_is_mac = 0;
if (grub_strcmp (fs->name, "hfs") == 0
|| grub_strcmp (fs->name, "hfsplus") == 0)
efidir_is_mac = 1;
if (!efidir_is_mac && grub_strcmp (fs->name, "fat") != 0)
grub_util_error (_("%s doesn't look like an EFI partition"), efidir);
(Prior to that, in commit cd46aa6c (2013-11-16), the grub-install shell script
was reimplemented altogether in C.)
