BuddyPress.org

Look at the bp_blogs_record_post() function in bp-blogs.php.

Are you sure it isn’t possible to hook into the “bp_blogs_activity_new_post_content” filter?
Using the $post variable, I’m pretty sure you can grab the $post->ID. From there, you should be able to use the get_post_meta() function and grab the custom field.

Append and return your extra custom field content to the “bp_blogs_activity_new_post_content” filter and it should work.

Though if Brajesh says he’s having trouble, then there’s probably another issue at work here.

If you’re using WPMU, then you’d need to use switch_to_blog() to grab the postmeta… which is quite intensive…