I just got back from a Taiwan trip, and I went to a restaurant inside an outlet mall where we had seen DCC before. Despite being by myself this time and my elementary Mandarin proficiency, I was able to opt out without any issue. I had control of the card and terminal throughout. The screen popped up "DCC..." and the currency selection was 1 for USD and 2 for NTD. The transaction has already posted as $16.31, so DCC would have been closer to 5% rather than the 4% quoted.