My question is: is bootable FAT32 support a possibility in the (near?) future?
Unfortunately, no. It's camera firmware limitation. But there is no problem to make 4GB card bootable. If the Mac does not allow you to format it in FAT16, find a PC and format the card there.
Have you tried using format.exe in DOS Box ?
man newfs_msdos
NAME newfs_msdos -- construct a new MS-DOS (FAT) file systemSYNOPSIS newfs_msdos [-N] [-B boot] [-F FAT-type] [-I volid] [-O OEM] [-S sector-size] [-a FAT-size] [-b block-size] [-c cluster-size] [-e dirents] [-f format] [-h heads] [-i info] [-k backup] [-m media] [-n FATs] [-o hidden] [-r reserved] [-s total] [-u track-size] [-v volume-name] special [disktype]
In this case, newfs_msdos is your friend
newfs_msdos -F 16 -b 65536 disk1s1