And fyi, here is the 3dVol2Surf command that was used to create the niml dset (which was then blurred using SurfSmooth):

# map volume data to the surface of each hemisphere
foreach hemi ( lh rh )
foreach run ( $runs )
3dVol2Surf -spec $surface_dir/PSK403_${hemi}.spec \
-sv ${subj}_SurfVol_Alnd_Exp+tlrc \
-surf_A smoothwm \
-surf_B pial \
-f_index nodes \
-f_steps 10 \
-map_func ave \
-oob_value 0 \
-grid_parent pb02.$subj.r$run.volreg+tlrc \
-out_niml pb03.$subj.$hemi.r$run.surf.niml.dset
end
end