I believe the hook you are looking for is right_now_discussion_table_end
Update:
From your comment i see that i did not explain myself, WordPress closes the table and div just after right_now_discussion_table_end action hook so you can use right_now_discussion_table_end and at the begging of your function close the table and div yourself and open your own, then leave your custom table and div open and let WordPress close them.