I believe a regular CX flyer (changi or mhtapai) said there was a way to do as the OP wants, but I have bad luck with the serch function in locating the link.
As stated before, only if one has a connection can you use the lounges. My question is that I have a 12 hour transit in HKG. I would love to take a shower at the lounge, then head into Central for a while, returning later for my same day onward flight. Any suggestions?