I don't know if this is still an issue for you, but I'll hazard an answer anyway. While you certainly could
write the MBR to your flash drive (and even use it as the boot partition), its not really a good idea. There are a few better ways of accomplishing your goal (suggested in ascending order of difficulty).
The first method would be to tell the openSUSE installer to not write an MBR at all, and use Super GRUB2 Disk
from a CD or your flash drive to boot the installation every time.
Although its fairly simple in theory, since the first method is a rather inconvenient way to achieve security, you may want to consider letting openSUSE configure GRUB as it normally would, then setting Windows as the default boot entry in the GRUB menu after installation. Restricting access to your openSUSE installation is probably best done the same way you would in Windows: setting a password on your user account.
If you are willing to perform more advanced configuration, maybe this third method will work for you. You could install openSUSE the same way you would in method 1: telling the installer not write an MBR. Then follow this
tutorial to add openSUSE to the Windows bootloader using EasyBCD.
If you are using either the second or third methods, you could add a password to your GRUB installation using the instructions found here
. I wouldn't recommend it, however. The best way to protect your openSUSE installation is probably by setting a password on your user account.
Use method 2 or just install Ubuntu using Wubi.