A word spoken rarely or never

Solution 1:

You could consider using archaic words which are:

These words are no longer in everyday use or have lost a particular meaning in current usage but are sometimes used to impart an old-fashioned flavour to historical novels, for example, or in standard conversation or writing just for a humorous effect. Some, such as hotchpotch, reveal the origin of their current meaning, while others reveal the origin of a different modern word, as with gentle, the sense of which is preserved in gentleman. Some, such as learn and let, now mean the opposite of their former use.

[Oxford Online Dictionary]

Archaic words might suffer the same problem as other suggestions you included in the question, but "rarely (never) spoken word" could be the right phrase.

Solution 2:

archaism [ahr-kee-iz-uh m, -key-] noun 1. something archaic, as a word or expression.

http://dictionary.reference.com/browse/archaism?s=t

In language, an archaism is the use of a form of speech or writing that is no longer current or that is current only within a few special contexts. Their deliberate use can be subdivided into literary archaisms, which seeks to evoke the style of older speech and writing; and lexical archaisms, the use of words no longer in common use.