Giving a talk on a paper whose authorship is not finalized
Z's name, I am sure, will be on the paper as an acknowledgement at least, even if not as an author. With that in mind, you could probably quite comfortably put Z's name up as a contributor without worrying about misleading the audience.

In some fields, this is made easier by a tradition of having an acknowledgements slide rather than putting up multiple names as authors on the first slide. In this model, only the speaker goes on the title slide, and everybody else worth mentioning goes in the acknowledgements slide (typically the last slide, which stays up during questions), with no particular distinction being made between levels of contribution. If this is accepted in your field it can give you a neat manner of sidestepping the question.