Some documentation:
1963 Constitution on the Sacred Liturgy, SACROSANCTUM CONCILIUM at
vatican.va/archive/hist_councils/ii_vatican_council/documents/vat-ii_const_19631204_sacrosanctum-concilium_en.html
“30. To promote active participation, the people should be encouraged to take part by means of acclamations, responses, psalmody, antiphons, and songs, as well as by actions, gestures, and bodily attitudes. And at the proper times all should observe a reverent silence.
31. The revision of the liturgical books must carefully attend to the provision of rubrics also for the people’s parts.”
So if people should do something, there should be a rubric telling them to do it.
From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from
romanrite.com/girm.html :
“42. … A common posture, to be observed by all participants, is a sign of the unity of the members of the Christian community gathered for the Sacred Liturgy: it both expresses and fosters the intention and spiritual attitude of the participants.”
So a special posture, an individual posture, a posture not in the rubrics, is at odds with this.
1997 Instruction ON CERTAIN QUESTIONS REGARDING
THE COLLABORATION OF THE NON-ORDAINED
FAITHFUL IN THE SACRED MINISTRY OF PRIEST at
vatican.va/roman_curia/pontifical_councils/laity/documents/rc_con_interdic_doc_15081997_en.html
Article 6, §2:
“… In eucharistic celebrations deacons and non-ordained members of the faithful may not pronounce prayers — e.g. especially the eucharistic prayer, with its concluding doxology — or any other parts of the liturgy reserved to the celebrant priest. Neither may deacons or non-ordained members of the faithful use gestures or actions which are proper to the same priest celebrant. …”.
From the GIRM:
“152. After the Eucharistic Prayer is concluded, the priest, with hands joined, says the introduction to the Lord’s Prayer. With hands extended, he then says this prayer together with the people.”