Unless I am really blind, it is impossible to have dynamic cross-references in an Impress document. It almost seems as if word-processing in Impress is not re-using the code from Writer. Dynamic cross-references are vital in Impress, because one so often rearranges the slides, and the hand-out version often needs cross-references. PowerPoint doesn't have them - please let LibreOffice be better!
Ok, I guess it is a valid request.