I don't think there's any definitive extropian position on the death penalty or any necessarily moral reason to object to it. If it is a useful deterrent then it should be no more controversial than any other means of reducing crime. This is in a society with limited government. Of course there are problems such as racial bias and convicting the wrong person etc but none of these should be given an infinite weighting under a proper rational economic perspective just as we don't give an infinite weighting to what *might* go wrong for other criminal penalties. One also has to trade these off against the lives which might be saved if the death pentaty were a useful deterrent.
What about in an polycentric legal order? I think there would be something resembling a death penalty anyway which undercuts the notion that you can decisively prove that the death penalty is objectionable on libertarian grounds. In a society based solely on private law, some hard core criminal types would be such a huge liability that eventually no jurisdiction would want to have them. They would be outlaws and anyone from any jurisdiction would be able to kill them without suffering any legal consequences whatsoever.