Here’s a clarification from Zenit‘s weekly liturgy Q&A column. While we’re waiting for Father Edward McNamara of the Regina Apostolorum University to chime in on the weighty issues of a choral Sanctus or the particular definition of participatio, here’s what he said about priests celebrating Mass alone:
(S)ome priests believe that this form of Mass with no faithful present is now forbidden. This is not the case. Indeed, present canon law, by requiring a just cause for celebrating alone, and no longer a grave cause as did the 1917 code, has actually made it easier to celebrate such a Mass even though it should always be seen as an exception and to be avoided whenever possible.
This seems reasonable. I’ve never understood the choice of celebrating Mass alone as opposed to being in a parish or other community. But why couldn’t a priest take a vacation in an isolated area, like any of the rest of us?
Catholic Sensibility 
Our former pastor was forced to step down because of an abuse accusation and was then forbidden to celebrate sacraments in a community. He told someone that he was celebrating mass alone daily at his kitchen table.
I think the more important reform of preconciliar practice was the move away from simultaneous Masses in chapels of churches, to move away from the idea that if one Mass is great, more at the same time must be Better. Other than that, if a priest is moved to celebrate Mass for a special intention (perhaps soon after hearing confessions or witnessing something meriting intercessory prayer) at a time or place where a mortal congregation is not readily present, that is laudable from my perspective, because it is the supreme prayer of the Body of Christ, and the heavenly court is ever-present.