It's likely you know more about canons and rubrics than I do (it wouldn't be hard to know more than I do, because this is an area of vast ignorance on my part!) But I was under the impression that a Pope's liturgical directions would take precedence over anybody else's, except for things that are strictly dogmata, e.g. the validity of the form or matter of a Sacrament.
Is not the Pope the chief liturgical authority of the Church? Does he not have this authority?
My concern is that a major deviation from such a clear instruction will give the green light to those who are inclined to invent a liturgy to be more inventive. Soon we would have a ghastly mess.
The HF is welcome to wash as many feet as he desires but on Holy Thursday it might be best to follow the rules and thus encourage Holy Obedience throughout the Universal Church.