Yes, Cartoon2D should be possible with GRHydro. Apart from certain technical difficulties (choosing correct coordinates, number of ghost points, etc.), the largest problem I expect is that Cartoon2D will interpolate the solution, and interpolation is not a conservative operation. Thus I'd expect problems near shocks, discontinuities, and surfaces -- problems in the sense of "the hydro scheme is inaccurate", not in the sense of "segfault" or "nan".

Most likely -- and I have no hard information on which to base my opinion -- what is necessary is a very careful debugging of your parameter file. It's difficult to explain this in an email (and I might also be wrong) -- feel free to call in next Monday for more in-person advice.

-erik