I don't know about mri_surfcluster, but I did find this info. The default should be in MNI305 space (the very original MNI space) by default now, but the options --fixmni and --nofixmni control whether you get TLRC or MNI space coordinate output. The header lines seem to switch between " TalX TalY TalZ" and "MNIX MNIY MNIZ". That program can be called directly or indirectly through mri_glmfit-sim to do cluster simulations for multiple comparisons correction. For FreeSurfer specific questions, the FS people will be the best to ask though.

[surfer.nmr.mgh.harvard.edu]
[www.mail-archive.com]

If you want to compute a centroid or some other center yourself of a cluster on a specific surface (for a subject or for a template), SurfClust can compute the cluster and a volumetric and a surface centroid.

For other ROIs or clusters computed for another surface, you can use the guidance here for the way it's done in the SurfIce software.

[github.com]
[afni.nimh.nih.gov]