Hi-

I split this question off and gave it a meaningful title because it is on a very different topic than what had previously been being discussed.

You are right, there isn't something in fat_proc_map_to_dti to handle surfaces. However, I asked the wise Daniel Glen about what function could *apply* a (affine, 12 DOF) transform made with fat_proc_map_to_dti, and he said ConvertSurface is the function in question. Now, I haven't used this yet, but you could:
1) use fat_proc_map_to_dti to align the T1w volume from FS to the DTI space, and then
2) take the transform made from that and apply it to the *.gii file surfaces with ConvertSurface.

This would be a really useful thing to include *within* fat_proc_map_to_dti, so I will probably aim to do that this week. If you want to try out the above, please let me know how it goes; if you want to wait until I have tried it myself and added it, then that's fine, too. I have just gotten up to this point in the online tutorial help for the fat_proc* functions, so this is excellent timing for this question-- thanks!

--pt