Maybe because on the GAL walks the other hounds scents are that strong because there are so many that he wants to leave his scent too and people legs are where other dogs have rubbed themselves, so are probably highly aromatic to the canine nose already and he is trying to mask it all with Eau de Zak.
I know you are probably at your wits end trying to stop him and i don't mean to be unsympathetic but i had to laugh when i read your post
Do you think it might be something to do with marking his territory when there are so many other dogs around? Although why he chooses peoples legs rather than the nearest tree or bush is beyond me Either that or there are an awful lot of people on GAL walks who's legs resemble trees!!!!
Either that or he just LOVES people (particularly me, Dave and Joyce)! lol
I shouldn't laugh but its been a standing joke between me and Dave cause the first thing Zak did when he met Dave for the first time after I picked him up and fostered him was to pee on Dave's leg at Strathclyde Park. But it is now getting beyond a joke ! I probably need to get a dog coat made up with " Don't stand still Zak will think you are a tree" or "Watch out I'll pee on ya " on it. At least that will warn people to stand at least a dogs length from him.
That's a thought! Will be fine as long as he doesn't develop a long-range style!
I know how embarrassing it can be though (being serious now) since my lovely Wills decided to pee on someones bag of groceries one day. I just wanted the ground to open and swallow us.