I was able to place blocks just outside the world border by placing it on the backside of a torch right next to the border. The block was taken from my inventory, was untouchable(as it was outside the border) and once the World border expanding into that block, I was able to mien and reclaim the block.

Place torch directly next to border. Right click with a block on the side of the torch facing the border. I would assume it to work with anything smaller than a full block, like panes, fences, and pressure plates.