While there are multiple instances of the Vatican discouraging it (and instructing priests to do so at every opportunity), Church documents have also reminded us that the Mass is living, not an old dead thing that can never change, but a living, breathing, adapting reality.
I’ve had it put to me this way, we are all “kingdom priests” that is, we all participate in the 3 offices of Christ as “Priest, Prophet and King” and so, while the Ministerial Priesthood (what we usually associate with the term “priest”) is the only one capable of some things like speaking In Persona Christi, we are all kingdom priests, and in the sense that we are called to offer the sacrifice, which means we are able and called to pray in participation with the Ministerial priest(s), (so we internally are praying what the priest is saying, but not in the same way, since only he is able to fulfill certain parts of it), and as such in the ways appropriate, and which we are ministerially able, we participate in the priestly duty. This means that we are called to offer the sacrifice (why we say “Amen” after the Doxology) and by this logic, we can participate by uniting our posture and his.
One may further expound this logic by noting that the GIRM no doubt doesn’t cover every minutia, making assumptions that we know enough, and customary practice will provide a guide to the details. I think it is reasonable to assume the GIRM doesn’t explicitly instruct the lay faithful to wear clothes, or when to kneel or sit or stand, this doesn’t mean that we shouldn’t wear clothes, and should never change from sitting to kneeling to standing.