Yes, I am. This also means that any open source distribution won't include those textures. However, if I do end up open sourcing the work I'll make sure to include instructions for people that already own Minecraft; the textures are just .png files that can be extracted from the game's .jar file and can then be transformed to be used on the FPGA.
I suggest you look at minetest[1] textures. It's an opensource minecraft clone. Most textures have CC or MIT licenses. Read license.txt for each texture mod before you use them. They may help in opensourcing your project.
I can't remember any off the top of my head, but I'm pretty sure there are a few open source texturepacks for Minecraft that would be a drop-in replacement (same filenames amd structure).