Hi there,
since I have this problem right now: Didn't figure out how to do this thing on a Macs terminal with diskutil or newfs_msdos (set the offset to 0) but I found that I can compile mkfs.vfat for the Mac.
Get the source from here and do the common tasks (cd to the extracted souce, then enter ./configure && make && make install).
Hope is is useful for somebody else...