Okay, before you judge me and scoff...I sometimes hike with my girlfriend. She loves the outdoors, but has not accepted the hat/buff look and she INSISTS on brushing her hair in the morning. Of course, I don't argue. =)
We purchased a tiny round brush at WalMart for $1. It was in the travel needs section, along with the travel size soaps, etc. Anyhow, it is about the size of a small makeup compact. It folds up flat. when unfolded, the bristles pop out and the handle is a round mirror. I don't know the weight, but it is not much at all. Maybe an ounce. Maybe less.
The ONLY justification I could muster was that it was multi purpose and small.
Certainly not as small and light as the Coghlan's mirror trimmed, but useful and it keeps her happy.