Theory: Brooke's music and (presumably) album will have become so successful that it will have travelled the entirely of the Grand line. In doing so, Crocus will get a hold of this music and recognise Brooke by his afro.
In his lyrics, Brooke will have messages for Laboon, telling him to wait a bit longer as he fulfils the original promise as the last remaining member of the crew. That is, Brooke became famous precisely so that he could leave a message for laboon while simultaneously spending his time honing his skills for the benefit of the Strawhats.
Huzza!