(progn (minibuffer-message "hello world") (print (format "current message: %s" (current-message))))
to get "current message: nil" in the *Messages*. If message is used in place of minibuffer message, works as expected.
The reason I think current-message should work with minibuffer-message is due to its docstring: "Return the string currently displayed in the echo area, or nil if none."
PS Is there some other way to get what's currently displayed in the echo area?