After noticing there were no rat artillery, I beleive I have found a conversion. You could get the Desert Rat HMG Team (http://i648.photobucket.com/albums/uu205/jchaos79/PEN/92.jpg)
then add peices of the Imperial Roman (or any other) ballista to it. Any thoughts, comments?
Could be an option. There's a few artillery pieces knocking about in the other ranges which could suit, the Dwarf one (FD11) would be a nice size for the Rats. Crewmen would be tricky, but you could use the Napoleon Rat for an officer and the large with spear and shield could be converted into a ramrod type. Not sure on one holding a cannonball, you'd have to get busy with the putty for that!
We can supply any of the individual pieces if you need them, just give me a shout.
8)
There is also the league of ratsburg cannon and crew.