To do this I'm going to need to access the video feed and before I go down the path of setting up the full Parrot SDK I thought I'd have a poke around the Sumo's Linux OS internals.
It turns out that the Sumo has yavta - aka "Yet Another V4L2 Test Application".
So, after telnetting to the Sumo on 192.168.2.1 I first needed to stop the Sumo process that was accessing the camera (disclaimer, this is the process that does "everything" that makes the Sumo work with your phone so you'll need to reboot to restore normal functionality):
kill `pidof dragon-prog`
Then I was able to take a snapshot using yavta:
yavta --capture=1 --file=/data/ftp/image.jpeg /dev/video0
To grab that image I simple FTP'd it from 192.168.2.1 - it's waiting in the root of the FTP file tree.
There's only one (not unsolvable, I hope) problem though - the frame wasn't fully captured so only the top couple of rows of pixels had valid data...