I'm working on a piece for Pierrot ensemble (fl, cl, vln, vlc, pa, perc). While it's not trying to be a strict or authentic tango, it uses several familiar rhythmic patterns and "sounds like" one. I like the timbre of castanets and am considering using them. I read that castanets are much more common to flamenco, but I also read that tango and flamenco are fairly closely related.
How out-of-place would it be to use castanets in a tango? Would it jar the listener out of the music too much?
Castanets are not used in an Orchestra Týpica in Argentina. However, they are common in German and American tango orchestras. Tango grew out of a combination of ballads and Bizet's "Habanera" from "Carmen." Flamenco styles are more closely related to Spanish (and Moorish and perhaps Gypsy) ballad styles. Dancing is a later addition.
Broadway tangos (like "Hernando's Hideaway) seem to always use castanets. I'd suggest trying it with and without castanets and see which sounds better (rather than the most authentic).
