I'd say that 'quoi' isn't really a partner of 'ne', like 'pas', 'jamais', etc. Certain verbs in certain situations form the negative with just 'ne' (savoir, pouvoir, cesser come to mind), and I think this is the case with 'importe' when it has this meaning. You can use it with other 'mots grammaticaux' like 'où' or the 'qui' in Spain_is_different's examples, but you can also use it with 'normal' words:
- Je prends une chemise. N'importe laquelle.
- Je le ferais, n'importe la difficulté.
- Chaque enfant, n'importe la race, religion ou pays, merite la promesse d'un avenir.
You can also use it on it's own:
- Il est interdit de faire ça ? N'importe ! Je le ferai quand même.