Ghillie shirts are silly shirts, unless you're in costume. Just wear what you would normally wear with trousers. I have been known to go to events wearing Iron Maiden tour shirts with my kilt! Remember, it's just clothing. I will wearing a costume shirt myself... but then again, I'm in the Renaissance Scots, and it's expected that members be in Renaissance attire.
"Two things are infinite- the universe, and human stupidity; and I'm not sure about the universe." Albert Einstein.