An application that do not rewrite the grub, but only update it on an specified partition

I always use two versions of Ubuntu on my machine: a stable OS and an alpha one. When I install the alpha version, ubiquity always rewrite the grub of Ubuntu stable and I have to restore and update the boot loader manually.
With my old PC, I used a floppy to obviate this problem, but now I do not have a floppy device. If I leave blank the device text box, or if I give a non existent device, it throw me an error and it do not clean the installation data.
If you make a grub-updater that do not rewrite the boot loader and/or that update the grub on a given partition, I can obviate this problem easily.

Also an fstab-updater can be useful.

Thanks for Ubuntu and for your support!