Topiary is a style and separate from bonsai which are never intended to look like topiary. These could be styled in the fashion of a bonsai but there is one big impediment: bonsai only works because we reduce the roots in order to limit growth of the canopy. You can apply the rules of bonsai to limit growth of foliage to a cloud layer growing ~sideways~only , not too high from the branch and none growing downward to create space between branches, keeping the profile in the form of a golden triangle as formal or informal upright, flat-top, windswept or what-have-you; anything except branches terminating in balls of foliage. It could look nice for a time, but you have to figure out how to limit root growth because a whole, intact root system would respond to the bonsai training as if it were attacked by Paul Bunyan or a hurricane and push growth, in spades. So, trench it to restrict, reduce and contain the roots and be prepared to keep after the top and the roots forever, just as you would something in a pot. If you accomplish that you'll be a hero.
