Say a company came to you that is currently suffering business loss due to a few negative online reviews. The company asks you to compose a series of reviews from a customer point of view singing the company's praises, only you have never been a customer. Would you take the job and basically just write fiction for them for business purposes?

No, I wouldn't endorse Pepsi because I hate it, and I wouldn't write up fictitious reviews for a site I've never had any firsthand experience to know if they're bastards or not.

No because that would be deception. I am not going to be passing my fiction off as true events. If people ever see a novel written by me in the stores/online (or see a movie/tv show, whatever), they're going to know that I intend it to be viewed as what it is, fiction. (Albeit fiction with a relatable storyline.) But writing fake reviews that I don't believe in? Pass.

I would need to try the product to see if it has any redeeming qualities. If I can find two or three to string together, I'd do it. If anything it would be more of a creative challenge.

Isn't that what a copy writer does? Most advertisments are written by people who have to connection to a product. My job as a copy witer is to increase sales / interest in a product.
Now if they want a personal endorsement from an unknown, I would definately need more information so I could possibly write positive press if that were my feelings..