It should be possible using photorec...
e.g. on my wubi install I did the following:
sudo apt-get install testdisk
sudo photorec /dev/loop0
[proceed]
[none] (partition type table)
P ext4 [search]
[ext2/ext3]
[free] (or I guess you could use [whole])
Then select some place to recover other than the wubi install (I assume you'd run this from a live CD, so select the local hard drive... I used /host/ubuntu )
Then hit Y to select and it runs.
Then check results. Here's some terminal output
bcbc@ubuntu:~$ sudo photorec /dev/loop0
PhotoRec 6.11, Data Recovery Utility, April 2009
Christophe GRENIER <[email protected]>
http://www.cgsecurity.org
PhotoRec exited normally.
bcbc@ubuntu:~$ ls /host/ubuntu
disks recup_dir.1 recup_dir.3 recup_dir.5 Ubuntu.ico winboot
install recup_dir.2 recup_dir.4 tedwireless.txt uninstall-wubi.exe
bcbc@ubuntu:~$ cd /host/ubuntu/recup_dir.1/
bcbc@ubuntu:/host/ubuntu/recup_dir.1$ ls
f0270448.txt f1666616.txt f1668496.txt f1791600.txt f2403160.txt
f0270472.txt f1666624.txt f1668528.txt f1791608.txt f2403176.txt
f0270488.txt f1666632.txt f1668560.txt f1791616.txt f2403208.txt
...
If you're running from a live cd, first mount your windows host and then the root.disk:
sudo mkdir -p /media/win
sudo mount /dev/sda1 /media/win #change /dev/sda1 for your own partition
sudo mount -o loop /media/win/ubuntu/disks/root.disk /mnt
Then you can proceed selecting the appropriate loop device.