s it possible for a Changeling to use their “Change Appearance” ability to produce the visage of a mask they’ve seen – but in fact, is their morphed face? Changelings cannot mimic the appearance of equipment or clothing; this is an important limitation on the power. So by default, no, a changeling can’t appear to be wearing a nonexistent mask. But everything’s up to the DM; if it’s an unpainted leather mask, I mightconsider it.
— Keith Baker (@HellcowKeith) November 27, 2019
“Changelings cannot mimic the appearance of equipment or clothing; this is an important limitation on the power.”
Yes, but the intent seems pretty clear.
A changeling shouldn’t be able to mimic a royal guard’s signature suit of plate armor and ceremonial spear, not only because that seems biologically questionable, but also because it trivializes the challenge of disguising.
Ordinarily you’d either need to obtain an actual suit of armor and spear to use (which won’t be easy, and which are only two elements of a full disguise) or you’d need to use magical means to produce an illusion (which can be seen through).
Recreating a mask, though? That seems pretty reasonable, assuming the mask is like most masks and conforms pretty closely to a “face”. It’s not like turning your head into an ornate plate helmet with strange geometries, or somehow extruding a spear or other long protruding object out from your hand somehow.
At the very least, I’d argue a changeling should be able to recolor their face in a rough semblance of the mask – reproducing the patterns and colors, but not the actual contours of the object. From a distance, or in poor visibility, it could perhaps be mistaken for the mask in question, but it wouldn’t hold up to a closer inspection. That makes it useful while still limiting its power. And it would, of course, still only be one element of a disguise – you might also still need clothes, props, a convincing voice mimic, et cetera.
Also, the player may not be using it for the purpose of disguise – they might be using it to show someone else what the mask looked like, perhaps asking passerby “Have you seen this mask?” or telling a guard captain “The cultist was wearing a mask that looked like this!”