Doesn't do image quantization or anything fancy, might still be useful if anyone wants to see what the code for implementing a converter looks like, or wants to add .SPR output to an existing Java program. I'll add features (fix support, multiple input file support, etc) as my needs for my homebrew project increase.
https://github.com/shicky256/ngcddev/tree/master/tools/neoconv/
https://github.com/shicky256/ngcddev/tree/master/tools/neoconv/