Greetings, Fr David. I was under the impression that the palm fronds held different significance. The fronds would have been used in the muddy or dusty bedrock roads to be laid down when a particularly important visitor came to the city (open sandals being poor repellants of dirt), thus protecting them from the dirt and grime of the road. The significance being that they would lay a soft bed for someone to trod on. Ergo, palms are significant because they’re a sign of Christ’s superiority… His position as king of the Jews. As an avid camper, I can tell you that pine boughs also make excellent barriers between a person and the ground.
Looking at this picture from national geographic:
ngsprints.co.uk/m147/Joe-Scherschel/p109330/Art-print:-Close-view-of-olive-tree-branches./product_info.html
I guess i can see how olive branches COULD possibly be used for the same thing, but not as easily. The large branches just aren’t the right shape for laying accross a road, and that would mean LARGE bulk of the smaller branches. Generally, given the historical symbolism of olive branches, I don’t think they would have been used for that purpose.

although I could be wrong about the practice that started palm sunday and the significance being derived from the involvement of the plants.