'Flattening' Surface Data

Version AFNI_24.0.02

Hello! I'm interested in finding a way to 'flatten' surface data that I have stored in a gifti onto an x/y plane in order to project a third value in the z direction. The aim is to fit a plane to that data, I would later want to bring the angle of that plane back into SUMA coordinates to visualize it with the GUI. I'm wondering if any existing AFNI program can help me with this, and looking for some general guidance on whether this is a feasible approach. Thank you in advance, it is most appreciated!