I have TMS data where I measured the hotspot for stimulation in relation to Cz on the participants' head. Is it possible in MRIcroGL (eg by drawing curvilinear lines and grabbing the measurements of these in mm?) to measure approximately where Cz would be on a T1 weighted scan? My ultimate goal is to see whether my TMS hotspots appropriately overlap with my 2x2x2cm3 GABA MRS voxels-- so the measurement has a little bit of "wiggle room" to work with. I would like to make some images where I show the MRS voxel overlaid onto the T1 + a mark showing the spot on the skull where we did stimulation. However, I'm stuck on how (and whether) I could get a good approximation of Cz in MRIcroGL?

MRIcroGL is a visualization tool, and is not designed for your problem. Given how the pulse will penetrate the skull, I am not sure a curvilinear correction is required. However, your estimation may be improved by modeling your coil design.