I run WS on a linux host. I would mount that virtual disk in my host, run something like
sudo dd if=/dev/sdX of=mbr.txt bs=512 count=1
where sdX is the VM's disk that you mounted
Copy it, hen use a hex editor to edit it, then do something like:
sudo dd if=mbr.txt of=/dev/sdX bs=512 count=1
BE VERY CAREFUL that you are using the mounted VM's disk, not your host's disk. Murphy is out there.............. waiting. 
BTW: this is the same way you can save/restore your mbr.
If you don't have access to a linux machine, you could probably do the same with a live CD but I am not sure if it would include the hex editor, so you would probably have to install one. I use UltraEdit, a paid for <gasp> editor.
I don't know about debugging, if it doesn't do what you want, restore the copy of the original
Lou
Edit: Oops, senior moment. This is what I do to work on a physical drive, not a virtual drive. My bad.