Dear Mary: What is the etiquette of greeting a friend who is engaged in a ceremonial display?

Q. Passing Buckingham Palace in a taxi the other day, I saw the ceremonial wing of the Household Division prancing impressively along on horseback. The taxi halted to allow them to pass. As I knew one of the young men socially, I wondered what is the etiquette of greeting a friend who is engaged in a ceremonial display? I can see it would have been disruptive to the integrity of the group if one member had to nod or wave back to somebody, but surely it would have been rude of me not to acknowledge him — even if only...