Assuming that conferences moving would even impact policy in the first place, I'd expect Las Vegas conferences to be the ones that trigger change. Just on the tech side, Black Hat/DefCon and CES immediately come to mind. Las Vegas relies very much on conferences, and at least one casino/resort owner (Sheldon Adelson) is a well-known Republican supporter.

Remember, though: This is still a low number of visitors being denied entry. It would take a sizable number of regular attendees—or a notable number of high-profile attendees—being denied entry to cause rumblings of change.