I studied Kenneth Slessor's poetry back when I did my HSC, although I focused on "Vesper-Song Of The Reverend Samuel Marsden" in my assessment task. However, I believe that the related text that I used could also be appropriate for "Wild Grapes" and "Out of Time". I used a novel called "The Wave" by Todd Strasser (pen name being Morton Rhue). If you have not read this novel before, it essentially tells the story of a history teacher, Ben Ross, and the nine days that he changes the face of Gordon High School by running an experiment to show his students how easily they could become Nazis. I believe that this would constitute good related material, especially if you wish to discuss nostalgia and regret.
Another text I would suggest is Hans Christian Andersen's book "The Little Mermaid". The story is quite popular and has seen numerous adaptations of different types. It essentially follows the journey of a young mermaid who is willing to give up her life in the sea as a mermaid to gain a human soul. You may wish to consider this book as I believe it would be good to use as related material. In particular, it would be useful to explore time and nostalgia.
I hope this helps!