Pleached trees are really just hedges on stilts. While most often ornamental, Jack deLashmet has used them to great effect to fill a space between 2 buildings in this exquisite courtyard.
And Peter Fudge has incorporated them in a really formal hedged garden in country Victoria.
The lovely Leigh from Brabourne Farm has a line of them in her wonderous garden in the Southern Highlands of New South Wales. When establishing your row, clear the trees of all the low branches for about 1.6-1.8 metres up the main trunk. You then need to be very diligent in pruning the canopy to form a continous elevated edge. Regular clipping to maintain its shape is mandatory, & once they are mature you'll probably need to scoot up a ladder to do the top. But I reckon it's truly worth the effort.