The Mesa Verde horses are not being gathered by chasing them into pens. They are being baited and moved by the subtle movements of people.
Having observers is not appropriate for that type of gather. They will not go to a baited pen with a human watching.
Maybe the park would allow you to put up a trail cam if observing is important, but keeping their world as normal as possible is very important for this quiet and peaceful process.
Please respect the need to keep the gather as low-stress as possible.
Counselor, New Mexico