Can Spotlight search for text within Safari for iPhone?
No, Spotlight doesn't search the current page from within Mobile Safari (but it does search Reading List and other saved/bookmarked pages) but you don't need a workaround since this function exists in Safari.

It used to look like this. 

On iOS 9, it's integrated with the main search field - begin like you're searching for a new web page and then scroll down to "On this page"



Once you tap that, you get this interface to pan to each instance of the search results with the term highlighted.