Twenty years after the first public release of Flatland the source code for Flatland 4.0 is now released under the MIT license! There are some libraries included in Flatland that are under different but compatible licenses. The scripting language Simkin is under LGPL. The jpeg, unzip and libpng are under licenses that read very similarly to MIT.
3DML and .block as formats are not patentable and are free to extend or modify as you or your company see fit. We will do our best to guide additions to the format over time to keep it relatively standard across implementations.
I look forward to seeing a 3DML revival out there of great new products based on this engine and 3DML/blockfile formats.