Question

If a floptical disk doesn't have a write-protection notch, how is write-protection accomplished?

Answer 1

It appears there is a "write-lockout" command in the firmware.
For more details, see:
http://en.wikipedia.org/wiki/Floptical

Answer 2

It does have a write protect tab a.k.a. switch. On the underside of the diskette. When you hold the diskette with the shutter away from you, it is in the near left corner. On a 1.44MB diskette, this is the location of the hole used to identify "HD" diskettes from DD diskettes.