Micher

location @api_v1 {
  include proxy_params;
  proxy_pass http://api_v1;
}

location @api_v2 {
  include proxy_params;
  proxy_pass http://api_v2;
}

location @mock_api_v2 {
  index index.json;
  root /path/to/dir/with/mock/files/;
}

location /api/v2/ {
  try_files @api_v2 @mock_api_v2 @api_v1;
}