This was one instance where the limited DR of Canon sensors was a major issue.
Given that the point of transition wasn't linear, I couldn't use GND filters. Also, there was a very gentle, almost imperceptible breeze. It was enough, however, to rustle the leaves on the vines, such that consecutive shots, taken for image blending, wouldn't line up. I took around 500 shots, and it was only by chance that I found a pair - 2 stops apart - that matched up.
With an extra 2 stops of useable DR, I could have just taken a single frame and just pushed the shadows.