Without arguing the merits and quality of this outing, I'll just note this...
After doing my best impression of Tennant shouting "What?!" at last three times upon Billie Piper's reveal, the first thing I considered was action figure merchandising. Now, maybe it's nothing more than hearsay, but supposedly Piper was rather dissatisfied with the mass produced version of her likeness, the defiantly posed figurine that came with Character Options' first major DW toy release, the 5 inch tall RC controlled "Battling Daleks". In fairness to the actor, it was NOT a flattering sculpt. Reports, well maybe just anecdotes claimed she refused to have any follow-up figures released. Obviously, this isn't true because Character DID eventually sell a more neutrally posed action with greater articulation and a better recreation of her features. Or maybe it was her dissatisfaction that resulted in the time that spanned between those releases, time that allowed for a renewed contract and personal approval of the design. Honestly, I don't know, and I don't really feel like engaging in the deep dive research that will require to either confirm or denounce. But if there is any truth to that alleged incident, I can imagine there will be some intense review of the figure design before it (or if it) gets approved for mass production.
Even if this is a case of short term stunt casting, odds are a figure will be sold. I mean, how much screen time did either John Hurt or Jo Martin have" Yet, they both got action figures. So even if it's just of a holiday special, or even a single scene before Piper's Doc sneezes or something and regenerates again, Character Options will want to immortalize her in plastic...again.