Some digging around points to the rules file being at present available at http://anonscm.debian.org/cgit/collab-maint/nginx.git/tree/debian/rules

Since the package-specific configures appear to inherit via the `$(common_configure_flags)` statement, modifying only that block may suffice:

 $ perl -00 -ple 's{$}{ \\\n\t\t\t--add-module=\$(MODULESDIR)/ngx_pagespeed} if m/common_configure_flags :=/' rules > nu
 $ diff -u rules nu
 --- rules 2015-12-17 14:49:12.000000000 -0800
 +++ nu 2015-12-17 14:52:52.171942309 -0800
 @@ -47,7 +47,8 @@
 --with-http_ssl_module \
 --with-http_stub_status_module \
 --with-http_realip_module \
 - --with-http_auth_request_module
 + --with-http_auth_request_module \
 + --add-module=$(MODULESDIR)/ngx_pagespeed

This solution treats the file contents in turns of paragraphs (`-00 -p`), and adds the necessary data at the end of each (and appropriate backwhacking, newline, and tabbery), but only for the paragraph that matches the `common_configure_flags :=` bit. To target each section (instead of just the common one), one would need to fiddle around with the `if` regular expression as appropriate.