Hi Michel,
The forward model used by FieldTrip is actually not used from Brainstorm but recomputed at each iteration of the dipole fitting procedure. It sets one dipole somewhere, computes a forward model for this dipole, simulates the corresponding EEG or MEG recordings, then moves the dipole in order to minimize the residuals (simulation - actual recordings), and computes everything again. The forward model is estimated at each step, possibly tens or hundreds of times. Computing a realistic BEM model at each iteration is not an option.
The only information we pass from Brainstorm to FieldTrip for the dipole fitting is the position of the spheres (the “head model” in FieldTrip). The actual forward solution is not used.
Francois