That’s a good point. However, I think this may be fixed by planning ahead. Get your friend/relative you meet to send the letters or encourage others to do so. Or heck, call up a bunch of people when you accepted to the show and ask them to basically spam the hell out of every possible service and communication platform with decoy messages.
You’d also ideally not be in one location too long, so the postmark would be outdated by the time anyone sees it.