Mounting a single partition dd image in macOS?
This doesn't answer the exact question, but the following has worked for me to mount a dd msdos image as read-only (-o ro).
$ hdiutil attach -nomount /Users/sunknudsen/Desktop/boot-clean.img
/dev/disk2

$ mkdir -p ~/Desktop/boot-clean
$ mount -o ro -t msdos /dev/disk2 ~/Desktop/boot-clean