When you set your book to perma-free on Amazon, the people who "buy" the book during that period are still considered a verified purchase if and when they leave a review simply because they clicked the buy button and grabbed your book. I have tons of verified purchase reviews for my book My Fair Assassin because it is perma-free.

I would set your book as free on all major sites like B&N, Kobo, iBooks, etc. and then once they are free, contact Amazon and ask them to pricematch for you. It helps to check and make sure that the book is free on all Amazon sites around the world. Sometimes Amazon will allow a book to be free in the USA, but it won't be free in India or the UK, etc.

Just contact customer service, be very nice and personable-these folks speak to mean people all day-and let them know you're running a short promotion and you would be so grateful if the book could be set to free on all Amazon sites. Include the links of your free books from other retailers to prove the price is free on other sites, and then wait for their response.

If you get a rep saying they won't do that for you then just keep sending in requests until you find someone who will. Always be very nice during the whole process.

Then once the book is free you can send your supporters links to the books on these sites, and once they read and post their reviews, the reviews will show up as verified purchases on Amazon.