That is a very interesting question. Even on OpenBSD which can be boot from the single floppy unlike FreeBSD it would be tricky. There are three different boot floppies for OpenBSD depending what kind of lets say i386 you want to boot. (there is a regular PC floppy, laptop floppy, and RAID floppy).
I would actually try something else. I am not sure if that can be done on FreeBSD though. OpenBSD provides something called cdbr which is the code for the first part of the boot process. It is only 2Kb. I would try to load that onto the floppy. cdbr will try to load bsd.rd (ram disk kernel) which contains about 5Mb of drivers necessary for honest boot. If you can actually specify that bsd.rd is on USB stick you will be in business.
As I said I am not sure if that can be done on FreeBSD but I am really curious to hear about your experience.
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