I want to set a nbt variable (or whatever they're called) as a TileEntity, is this possible? or would I have to store the x/y/z coords of the block.

You can't save a tileEntity by itself, however you can do this:

 

SomeTileEntity tileEntity = ......;

NBTTagCompound compound = new NBTTagCompound();

tileEntity.writeToNBT(compound);

 

Then later, to get the tileEntity back

NBTTagCompound compound = //retrieve the compound

SomeTileEntity tileEntity = new SomeTileEntity();
tileEntity.readFromNBT(compound);