The num and den inputs to filtfilt() are for a discrete-time filter. But besself() returns the numerator/denominator for a continuous-time filter.
If you want to filtfilt() the signal with the Bessel filter, you'll need to transform it to a discrete-time approximation using methods such as bilinear() or impinvar() in the Signal Processing Toolbox, or others avaialble in the Control System Toolbox. Keep in mind that the magnitude response with filtfilt() is the square of the magnitude response of the filter being used, so the magnitude of the discrete-time approximation is what's important.
If you want to do the forward/backward filtering with the continuous-time Bessel filter, I'm afraid you'll either have to write your own code or perhaps use Simulink to solve the ODE with sig as the input, then go in reverse. Of course you'll then have to decide on how the ODE solver is to compute values of the input signal between the samples in sig (assuming use of something higher than a first order solver). Also, filtfilt() takes some action to deal with transients that may have to be considered if writing your own code.
