Just got one
We just entered a jib and main series here on the Long Island Sound, and to race, a sail number was required. The local sail makers told us they'd put anything we liked on the sail, but the idea was to be unique in any events we might enter. They recommended the hull number, making it highly improbable that another boat would carry the same number. As well, they said, no problems would arise with race committees of more serious, sanctioned events. Seems to be the accepted way to go.