After noticing there were no rat artillery, I beleive I have found a conversion.  You could get the Desert Rat HMG Team
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.

There is also the league of ratsburg cannon and crew.