I am also not a hammock user, but a guy needs privacy only to change his pants. I would expect that one could wriggle in and out of a pair of shorts without too much difficulty. Otherwise, a quick trip to a nearby restroom should work.

Other ideas:

1. A quick trip into the woods (if any).
2. Don a poncho and change shorts underneath it.
3. Wait until nightfall and change under cover of darkness.
4. Wrap a towel around your waist and swap pants underneath.
5. Wait until nobody is in sight and be quick about it!