You can actually do partial redemption with Capital One as well. I've done it about half a dozen times with the Venture Rewards card. One way that I know for a fact that I've done it is to specify that there were multiple airline tickets purchased and only get reimbursed for "one" of them. It's just a little drop-down menu.
For example, you can specify that a $100 travel expense on your bill was 10 airline tickets and get reimbursed for only one at $10. It's a self-serve web interface. Help yourself.
There may be other ways as well.