I know that libx265 supports high-bit-depth. I should've chosen my words more carefully. What I meant was: the libx265 that's part of my FFmpeg binary doesn't support high-bit-depth, because...
http://hg.videolan.org/x265/file/tip/source/CMakeLists.txt:
Code:
if(X64)
# NOTE: We only officially support high-bit-depth compiles of x265
# on 64bit architectures. [...]
[...]
endif(X64)