I had a similar requirement using 2013 & XPM (I am assuming it is the same structure), and after trailing through JavaScript files and decompiling several DLL's I found that this list comes from some SiteEdit 2009 leftovers.
First the code looks for a registry key specifying the location of the SiteEdit config file. This did not exist with my XPM install, so I created it.
(64Bit Key)
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Tridion\SiteEdit2009
SiteEditPath: C:\Tridion\SiteEdit 2009\
This key then tells the dashboard where to look for the config file, which contains the list of Editable sites
<siteMappings>
<siteMapping proxyPattern="http://staging.domain.com" stagingUrl="" excludeHeaders="Authorization|Content-Type|If-None-Match" />
</siteMappings>
Hope this helps