Hello Matilde,
You could KIND OF do it if you mapped data onto standard meshes and used 3dGroupInCorr on the surface. Though hidden from view, the generation of standard meshes does involve non-registration of sphereical representations of the cortex, however the process is data independent so you can map single subject data onto their standard-mesh surface in original space then proceed with 3dGroupInCorr.
If this sounds too confusing, take a look at @Install_InstaCorr_Demo , one of the demos is 3dGroupInCorr on the surface. We can take it from there.
cheers
Ziad
