I think that it is very much in bad taste. This behaviour is incredibly offensive, especially given there are many other places in which you can piss such as a urinal or Margaret Thatcher’s grave. Your choice of doing so in Boer skulls shows a lack of remorse for killing them; no wonder then that Boers in South Africa call for international recognition of the white genocide they believe that they are facing, despite denial from official and mainstream sources so far. This question is much more complex than you give it credit for, and I believe you should research more on the topic, taking all sides into account, before thinking of making such an offensive joke again.