Generally speaking, the more formal an outfit, the sleeker it is.
I suppose this is true, generally speaking. But it seems that Highland rules sometimes tend to veer from that path. Plain-colored hose for daywear, but diced/patterned hose for formal occasions. It seems really strange to my decidedly non-formal mind that one would put on fancy hose and then kill the effect with plain, boring, generic-looking shoes that look like they were made by someone with absolutely zero imagination.

While the plain black shoes you referenced might be appropriate, one has to ask: do they really "go" with Highland attire from an aesthetic perspective? Seems to me that buckle brogues are perfectly appropriate for even the highest levels of formality, and lend an air of class and style that those plain black shoes simply cannot.