Comment 1 for bug 252900

Since link it isn't a fat32 supported operation I found a workaround : if you have an external disk or a partition which have a linux filesystem it is possible to copy the wubi-ubuntu /boot/ directory in that linux filesystem, mount it as /boot/ in ubuntu, upgrade the kernel and then remount it in another mount point (/mnt/tmp/ ) and copy the new boot over the ubuntu "old" one, then reboot the machine.

If it isn't possible to have a dedicated temporary boot partition formatted with a linux file system you could always make a temporary boot directory over an existing fs and then export via nfs that dir and mount it as boot.
Keep in mind , if you use nfs that you need no_root_squash as an option to make apt-get assign correctly the ownership of the files.