Tried it once, long ago (from what I'm told). Parents would use a taller child to get the band, cut it off, tape it to the smaller child, then get another band for the taller child from another attraction.
There is a "shoes count" rule when measuring a child's height - in other words, the shoes count towards the child's official height - CMs may not ask a child to remove their shoes before being measured. However, I've seen too many parents lately bring their kids in wearing platform sandals with four-inch soles. And you can just see the discomfort in the CMs faces as they're forced to say, "Well, the rules say the shoes count..."
The only real solution to the issue is to measure the child each and every time. And for God's sake, folks... if your child is too small, do NOT make the situation worse than it is. CMs do NOT enjoy turning guests away from the attractions. If your kid is too small... DON'T BRING THEM TO THE ATTRACTION. You have a TAPE MEASURE AT HOME! Measure your kid and find out FIRST what he or she can and cannot ride! If they are too small, comfort the child, tell them they were brave for trying, and that there are plenty of other rides at Disney.
I lost all sympathy for guests with too-small children the day I personally saw a mother tell her obviously too-small child, "Ok, now remember - wait until the man tells you you're too small to go on. THEN you can start crying."