How to write MBR recursively?

[Michael Spinnenhirn]

Hello,

I have the problem that my root-partition is mounted at /tmp/root. In a script 
i copy a hard disk image to this directory and now i want to write the MBR 
recursively to /tmp/root. How can i do this in my script?
The option --root-directory doesn't work properly. grub-install also doesn't 
work because grub-install calls grub which set the grub variables to /sbin/grub 
and so grub couldn't be found.
When i call the grub shell directly in my script i have the problem that this 
opens a subshell and all the commands after quiting the grub shell aren't 
executed.

Any help would be appreciated.