I've ended up with the boundary of my quarry such that the corner Land Mark is at point (0,0), i.e. not in the same chunk as the quarry machine.
Will the quarry's chunk remain loaded?
Yes, it will. I did some experimentation, here is what
/chunkloaders looks like after I've placed the quarry:
/chunkloaders is a command added by ChickenChunks that pops up an extra window showing you which chunks are loaded in the world. White is not loaded, red is loaded, and a green dot means force loaded, by a quarry or chunkloader block.)
As you can see, the extra green dot hanging off the edge is the chunk with the quarry machine in, it is loaded on top of the area within the Land Marks.