Pagespeed Javascript Minification / Concatenation - Add Banner and Footer
Since the question is still not receiving enough attention, may I suggest you look into the sub_filter and add_after_body directives? It would let you change one string with another, or add results of processing another location (which could be an internally-generated page, with the help of return 200 "body to return"). These modules are already included with nginx, but is not built by default.