Having been geocaching for a number of years, mostly in my P85 for the last few years, I almost always just rely upon either my handheld GPSr or smartphone for the entire trip, given that one can't readily upload a bunch of waypoints / POI into the S nav system.
For one geocaching event I attended perhaps similar to yours (where I had to drive between 24 different locations spread around a large area within a certain timeframe), I manually input the coordinates so that they were at least in my nav history to quickly switch between. That pre-dated the ability to tap to add a destination at an arbitrary point on the map, which would have greatly simplified things, as I was basically doing what you plan: Use the S nav system to get to parking, then switch to something more meant for going off-road.
edit to add: One gotcha I've encountered with passengers attempting to use their handheld or auto GPSr in my S; some of them apparently can't get a decent enough signal through the windshield shielding to be functional.