This is a question I've been thinking about a while now. If a Metamorphmagus doesn't have a set form, and someone takes their hair for a Polyjuice potion, what exactly happens to the person drinking the potion? Do they turn into the form that the magus had at the time, or to the original way that the magus looked before their form started shifting?
Polyjuice clearly copies the current appearance of a person - for example, fake Moody copied the injuries, including the missing eye, of a real one. Haircuts are copied as-is (despite hair growing in all wizards as a dynamic thing). Harry's scar was copied in Battle of Seven Potters.
As such, it's more likely that MM's current form when the hair was taken would be copied, since their ability actually change their physical form.
Warning: this contains a lot of 'maybes'
So for this to work, you'd have to take a hair from a metamorphmagus. This implies that they would need to be in a form that had hair. It is logical to conclude that, assuming it worked at all (remember what happened with Hermione; it's really only designed to work for people) it would take the form of whatever the metamorphmagus was in, because the polyjuice potion makes you look like what the hair's owner did. As the hair is that of something with hair, you would probably end up turning to look like that creature or person.
Importantly, bear in mind that in the movies when Harry and Ron took the Polyjuice potion, their voices didn't change which suggests to me Polyjuice is restricted to appearance only! Yet, as pointed out by Mooz in the comment below, in the books, the Polyjuice Potion seems to change one's entire biology. Now I would still argue that it would only change one into whatever form the Metamorphmagus was at the time, because one is taking a hair from whatever form it was in then. To my mind then, it seems most likely that you would take the form of the being the Metamorphmagus was at the time, rather than assuming the other properties of the Metamorphmagus.
Hence, assuming the Polyjuice Potion worked at all (which is doubtful as we've seen with Hermione), one would probably take on the appearance of what form the Metamorphmagus was at the time the hair was taken.