What is the ferry range of an A380?
You have asked several questions. I'll try to answer them in order to clarify.
Is the ferry range specified anywhere?
From the Airbus website, we find for the Engine Alliance GP7000 engine option the ferry range is ~9,600 NM. Let's just say it's 9,500 NM.
A: 9,500 NM. Also, waiting for the right trade winds can increase you range.

Can an A380 fly between any two airports on Earth?
If you place the originating marker over Dubai, UAE, then the A380-800 can fly to any airport on Earth except a region in South Pacific Ocean. This area (a circle of around 6500 km diameter) has many small island states (French Polynesia, Cook Island, American Samoa etc.).
(gcmap.com)
A: Yes, probably over 99% of airports.

Has a flight by an A380 in excess of 15,700 km been performed to date?
None that I could find. The longest flight of A380 (currently) is QF 8 (with duration 16:50 hours).
We know that A380 (October 25, 20071) is much newer than 777 (June 7, 19952) and 747 (January 22, 19703). In future, there can be attempts to fly it in excess of 15,700 km.
A: No record so far.

What is the ferry range of an A380?
This was your original question. Most of the flights mentioned on the Wikipedia article you referred to were either delivery flights or for demonstration purposes. For such flights, you can have:

lesser people on board
lesser equipment (seats etc.) so more space to add extra fuel tanks

With the above two changes, the result would be:

Less people = Less weight. The aircraft will weigh less and its range can increase.
Less seats = Less weight. The aircraft will weigh less and its range can increase.
More space to add fuel tanks. The aircraft's range will increase.

In addition, A380-800 has cargo volume of 6500 cubic feet. If we have a hypothetical rubber tank which fills all this space, we can add 184,000+ L of additional fuel than the original capacity of 323,546 L. This can increase the range over 50%.
This website contains pictures of additional fuel tanks. As you can see, those are inflatable and can adjust to the space available.
A: It depends.

1: Introduction with Singapore Airlines
2: Introduction with United Airlines
3: Introduction with Pan American World Airways