A friend and reader writes:
According to Canon J.B. O'Connell citing an instruction by the Sacred Congregation of Rites (forerunner to today's Congregation for Divine Worship and the Discipline of the Sacraments) dated "January 28, 1920, for the celebration of Mass by a priest who has lost his right arm, and has obtained a dispensation from the Holy See to say Mass [the dispensation might not be required under the current code of Canon Law, comments my friend]: ... (3) The celebrant is to make the cross with his left hand on himself ... in the usual Latin manner, i.e., tracing the transverse line from left to right. (4) The celebrant is to keep his hand laid flat under his breast ... whenever he ought, were he not disabled, to hold his hands joined before his breast... However, if with an artificial right hand he can observe the rubric becomingly, he does so." (The Celebration of Mass, pg 381).
I'm grateful someone's figured this out, as it's less work for the rest of us, which is part of why the Catholic Church is so great, really. You never know when you just might need it.