Same dedicated Windows drive for Boot Camp and VirtualBox?

The answer is No. Sorry. I have done this already. Here is the problem. VirtualBox and your Apple use different hardware. If you try to switch between the hardware you are going to drive windows insane.

If you first install Windows using VirtualBox's virtual hardware and a physical disk partition, then switch to Apple's physical hardware with the same physical disk partition then you have to run Microsoft's "System Preparation Tool". This tool is meant to be run once, not repeatedly. See the procedure at this link

The other problem would be now are you going to activate the software for both machines. Are you going to active while running under VirtualBox or BootCamp?

When you boot between the physical Apple hardware and the virtual VirtualBox hardware, the Windows operating system is going to detect the change. For example, the two machines do not share the same MBR. Each MBR will have a different signature. Windows stores this signature in its boot partition and compares the value with the MBR when booting.

While VirtualBox is capable of Raw Disk Access it is not designed to use the Windows Boot Camp partition as a virtual machine. In other words it does not handle the Windows Product Activation between running it physically and virtually. VMware Fusion, a paid product, is capable of seamlessly handing using the Windows Boot Camp partition as a virtual machine.