You do understand that doing that constitutes defamation of the ballot causing the vote to be thrown out don't you?
That's the point. I've also heard it called "Mickey Mousing" after one of the more common write-in options. You're Australian, and I am given to understand it's illegal there. However, it's not illegal in the USA, and it is a perfectly valid method of protest.
That said, Bork², voting for Trump can in fact be a protest vote. A protest vote is any vote undertaken to demonstrate one's dissatisfaction; it is not solely limited to spoiled or blank ballots. Someone who loathes Hillary might vote Trump not because they support Trump's policies, but as a demonstration of their dissatisfaction with Hillary's nomination. It, in my opinion, is a rather foolish way to protest-vote, as the post-referendum Brexit uproar should demonstrate quite vividly, but it is still a protest vote. In my opinion, an optimum protest vote would either be an outright spoiled ballot or, preferably, a third-party candidate that matches your political leanings (Libertarians, Greens, Reform, and perhaps even Constitution, though I'm cynical enough to suspect Trump will be absorbing most of their votes); it sends a message to the big tent parties not only that you're dissatisfied, but the direction in which you want them to go if they want you back. Plus, you won't be kicking yourself if you actually, by some mischance, somehow win against all odds.
EDIT:
That said, you're quite right in that it's unlikely to be
seen as such, unless it happens frequently enough to sway precincts bluer than a...uh...very blue thing. Doesn't change what it is, but it changes very much the practical effect on the Democratic party leadership. Another reason not to cast a protest-vote for Trump, I suppose.