Owners definitely are responsible for sedating if they want heads and ears clipped on an uncooperative horse. Dormosedan or some acepromazine + xylazine are good options, but definitely insist that owners (or the properly authorized barn manager/trainer) take responsibility for this aspect of things. A twitch may also be necessary.
Additionally, the ears are a very sensitive spot, so always be extremely careful to use sharp, cool blades and a smoothly running set of clippers. Being meticulous about these details gives a horse every chance to get better about this procedure instead of worse.
I would definitely discuss with owners what they can do to help their horses get less sensitive about being clipped. Simply handling the ears frequently helps a lot, as do gently holding the clippers against the ears for brief moments when touching up bridle paths, etc.
For your own horse, clicker training can work wonders for this type of thing. But, I think few owners would bother to follow through with it so I’m not sure it’s worth you recommending.