Did you already check at http://serverconfigurator.intel.com/configure-ap.aspx?id=MTY2NCMxLDI1MTcjMSwyNjE4IzIsMjUzMyMx&ShowNavigation=false ? That should have the complete list of peripherals that work with that board.
Even if someone else found an eSATA that worked once with the S5520HC, that doesn't mean it would work reliably all the time. I would stick with just the tested hardware.
That is the first place I looked and I also called support and there is no validated eSATA on the list, that is why I'm asking is anyone has had any suggest with one.
After testing an number of eSATA adapters I found that none that I tested worked if they had the Silicon Image chipset.
However I only test one with the Marvell chipset and it worked. It was the Rockett 622A Dual Port eSATA 6Gb/s PCI Express 2.0 host adapter.