to avoid this "ugly" attribute I think I would prefer to have 2 functions calls

helmet_map = mne.make_surface_mapping(evoked.info, helmet_surf, 'meg', trans, n_jobs=-1)
evoked.plot_field(time, helmet_map)

helmet_map would be a dict with surface coordinates, subject name and the API of plot_field
would be simpler.

what do you think?