There is no way that they can force you to do anything against your request, its grounds to sue. there are so many good reasons to allow the cord to pulsate and placenta to come away in it's own time.

You could always try negotiating with the Ob or changing Obs if it means that much to you. Find out why she/he doesn't allow it and if she's/he's worried about bleeding, then ask if you can have the injection if you do seem to be losing alot of blood. Obviously you would be wanting to avoid induction or augmentation with synto which makes it more likely to bleed and need a managed third stage. But you have the power, you are the 'customer' and you should get the service you have paid for. I think some Obs forget you are the customer.