The short of it is you really can't, at least not without some serious hacking of flyspell and/or your major mode.
From the flyspell source
(defun flyspell-prog-mode ()
"Turn on `flyspell-mode' for comments and strings."
(setq flyspell-generic-check-word-p 'flyspell-generic-progmode-verify)
The relevant line being where we set the
flyspell-generic-check-word-p function to
flyspell-generic-progmode-verify that function looks like this
(defun flyspell-generic-progmode-verify ()
"Used for `flyspell-generic-check-word-p' in programming modes."
(let ((f (get-text-property (point) 'face)))
(memq f flyspell-prog-text-faces)))
Finally we need to look at
'(font-lock-string-face font-lock-comment-face font-lock-doc-face)
"Faces corresponding to text in programming-mode buffers.")
So as can now be readily seen, flyspell is using the fontface of the text to verify what it has been typed as ( this is actually probably the most efficient way for flyspell to handle this problem. It means that it doesn't have to re-parse your code ).
Your options are either a custom version of
flyspell-generic-progmode-verify to load into
flyspell-generic-check-word-p or modifying the major mode such that it produces different faces for your various strings.