What I gather from reading many sources by now is that an individual BUK missile launcher's radar will not display civil aircraft transponder codes on a stand-alone basis. In order to display such codes the individual launch vehicle would have to be tied into either the ATC system's radar or a central military radar with ATC secondary radar capabilities.
Any person can acquire and use a secondary transponder receiver, freely available over the internet to purchase. It would be a standalone unit, and not integrated with warfare equipment, but available, yes. Would they use such equipment? Would make sense, but doubtful.