Alex, I'd started typing a reply and got sidetracked, but my thinking was similar:
What about extending each post up to 15 feet, with a row of panels running along the length of row ,15 feet above? The machines would have free movement between the rows, but could a machine be invented that could still straddle the row, but somehow navigate the posts sticking up?? Not sure the yield from the panels would be worth it of course.
In Argentina, one viticulturist I visited was experimenting with different colours and materials of netting. he nets anyway to protect the fruit, but he thought there could be means of acheiving solar gain or reduction with net colour, material and mesh size. Some future tech might even give them solar panel tech.
The idea of having two crops in one field was common in small vineyards in Portugal and other subsistence farming communities: the grapes grown on high pergolas, with a second crop, maybe maize or wheat, planted beneath. So there is something to double-decker thinking. Obviously you'd lose the benefit of the shade by reversing this for the solar panel idea - pergola trained vines above the solar panels - but good quality could be acheived and some genius could invent means of being to tend the vines and allow the panels to perform.