PCED's response is consistent with its declarations over the years on who may fulfill the role of subdeacon at a Solemn High Mass.
This consistency respects the nature of major orders and the clerical state, sounding a theme that it is not licit for a random layman to fill in for clergy - even if the substitution is going to result in a visibly and/or audibly more beautiful liturgy. Liturgical law must prevail over even a well-intentioned effort to improve quality.