Buddypress sitewide activity filters

Hi

On the sitewide activity page, rather than the dropdown letting me filter on posts, comments etc. I want to have "Posts" and "Comments" as tabs in the activity page menu, linking to /activity/posts or /activity/comments - so a new page that shows the filtered sitewide activity rather than using AJAX. Any idea how I do that?

I've managed to create the menu links by hooking into bp_activity_type_tabs but the pages /activity/posts and /activity/comments are blank (they show the site header but no page title or content). How do I get those pages to display the filtered activity?

I've managed to get this to work in the personal activity stream using bp_core_new_subnav_item to create subnav items under activity with their own screen functions, so thinking maybe using bp_core_new_nav_item but can't see how.