Thanks all of you @encukou, @michael_neel and TilemapKit for the detailed description about open source management. I see you all agree. It’s similar to administering corporate projects, not everybody has rights to push commits to the repository. In fact Git is still more strict than Subversion (where I manage company projects), because commits must be reviewed before being accepted. In SVN any user with write rights can commit without supervision. So this is good news.
Nobody has attemped this. The usual approach with GPU is render each tile and sprite as a quad composed by two triangles. Tilengine draws horizontal lines, not quads. But it may be implemented as decomposing the tileset in many 1D textures, and the tilemap and the palette encoded as textures passed to the shader and generate line primitives from the main program… of course there are challenges, (especially in affine transformed layers) but challenges is what drive developers! 
The rendering code is totally platform agnostic, what needs to be ported is the makefile, compiler and linker flags… But having the linux working, and knowing that OSX is FreeBSD based (Unix), I think that the differences may be small.