Comment 33 for bug 653134

@Colin,
I don't really understand enough about what you are trying to accomplish... is it that you have to update the wubildr? I ask because I've never seen wubildr updated by a grub-pc update on any wubi install I've had, despite the fact that grub-install has the ability to do this.

If the problem is that you need to find /host/wubildr to identify a wubi install, why don't you instead use /host/ubuntu/winboot/wubildr because this is always present on any wubi install - i.e. change /usr/share/lupin-support/grub-mkimage
Note if you do need to update wubildr it could always update the /winboot copy if there is no /host/wubildr and then just prompt a user to boot into windows and copy \ubuntu\winboot\wubildr to c:\wubildr. It's a bit cheesy, but it's better than leaving a wubi install broken. The only alternative would be to iterate through all partitions and mount and update any /wubildr it finds - but this might be considered too invasive.