I don't believe there is a way to add it to a custom watch face.
It is worth noting that the "No Fly" is simply a 24 hour countdown timer so it really doesn't tell you anything you can't work out for yourself.
For example, if you finished your last dive at 11:30am, the "No Fly" will count down until 11:30am the next day. For me, it is easier to remember I can't fly until 11:30am the following day than it is to keep checking a timer which is going to end at that time anyway.
Now, if you don't subscribe to the 24 hour rule of when you can fly (some people go with as little as 12 hours for a shallow dive, while others may go for 36 hours after multiple deep dives), the "No Fly" is of little to no value. Surface Interval would be a much more useful metric.
