I don't think nonword works. My first reaction was that nonword makes no sense in English. A nonword isn't a word, so if it's a word, even a detestable one, it can't be a nonword. According to Merriam Webster, however, that is one possible meaning of nonword.
Definition of NONWORD: a word that has no meaning, is not known to exist, or is disapproved
The Wikipedia page for pseudoword makes more sense to me.http://en.wikipedia.org/wiki/Pseudoword
"Vonk" is a pseudoword in English, while "dfhnxd" is not. The latter is an example of a nonword. Nonwords are contrasted with pseudowords in that they are not pronounceable and by that their spelling could not be the spelling of a real word.
To me pseudoword or nonsense word make more sense for the first meaning in the Merriam Webster definition, and I am unconvinced that nonword is used in the third, Unwort sense of a word that is disapproved.