So far as I am aware, there is no hook in the location you need, if I understand you. The closest I can think of is loop_start, which as you might expect runs when the loop begins. The problem with that hook is that you don’t know what content the theme may have between the <head> of the document and the place where the Loop runs.
By far the best solution is to edit the functionality, or your own hook, into your theme if you’ve written the theme, or create a child theme and edit that if you are using someone else’s theme.