Although, I dis agree with their view, I refer to one of the certification bodies and theyadvised me that it can be applicable and can be certified to ISO9001.
Certainly, if you ask enough CB's, you will find one that will encourage you to attain certification of a single department. After all, you are a revenue opportunity for them.
I work for a very large, international CB, with over 70,000 management system certificates issued around the World, and would tell you: It does not make any sense whatsoever for an individual department of an organization to attain certification to ISO 9001. You will create artificial controls and procedures, attain certification, but the organization will see no real benefit.
And, in the unlikely event that your external clients are knowledgeable enough, certification of an individual department might even be detrimental to your organization, because an observant customer, when faced with your certificate would ask: why would this organization certify a separate department only?
Paraphrasing NIKE: JUST DON'T DO IT!