Moonlight|3D is open source under the GNU Lesser General Public License version 2. The complete source code for the program is available on the download page and you are free to modify it and distribute your modifications as you see fit.
There is a subversion repository hosting the most up to date development version of the source code at
http://www.moonlight3d.eu/svn/Moonlight
A Trac instance with documentation for developers exists, but is currently offline. It will be brought back online as soon as possible.