Just to bring this thread to a conclusive end
here's the answer from Amazon, if anyone is interested:
"You can certainly publish your book for Kindle singles. There are no restrictions to any publishers from publishing for the Kindle singles program.
Please find more information regarding publishing your book on Kindle singles here:
1. Publish your work in the Kindle Store using Kindle Direct Publishing (KDP). The tips below should help you get started:
a. Watch our "How to Publish to Kindle" video, which shows a real-time experience of publishing a book:
http://www.youtube.com/watch?v=ltScSWVLQLk
b. Read our step-by-step publishing instructions available at
http://forums.kindledirectpublishing...?externalID=33
Please contact
kdp-support@amazon.com if you have any questions with the publishing process.
2. Once the book is published in the Kindle Store (usually takes around 24-48 hours), please send the below information to
kindle-singles@amazon.com:
a. Your KDP account e-mail address
b. The published book’s ASIN. The book ASIN can be found in “Product Details” section of your book in the Amazon Kindle Store.
If your work is accepted as a Kindle Single, you will be notified by email about any other further steps, and your book will be made available in the Singles section of the Kindle Store. Even if your work is not accepted, it will still be available to all Kindle customers in the Kindle store for as long as you make it available through your KDP account."