Somehow I didn't manage to find it in my searches earlier today, but I've just found this thread asking almost the same question, which has the solution.
system.tag.writeAsync([self.view.params.tagPath + '/Start'], [1]) should do the trick, I think. I'll try it in the morning.
Still interested to hear if anyone has a simpler way of doing indirect tag addressing on a button
(edited to add square brackets, as required for system.tag.writeAsync() function
