Sorry, but i just did a test
@Override
public boolean onBlockActivated(World worldIn, BlockPos pos, IBlockState state, EntityPlayer playerIn, EnumHand hand, EnumFacing facing, float hitX, float hitY, float hitZ){
if(worldIn.isRemote){
Minecraft.getMinecraft().displayGuiScreen(new HUDEditorGUI(playerIn));
}
return true;
}
Works fine in single player, but as you predicted, fails in dedi servs, so since he is using for SP, it assume it should work fine?