ENH: Expanding functionality for mixed source spaces and source estimates

You should be able to use rounding and your top notch coordinate transform information to do this easily. The surface ['rr'] is in one space. I would just transform the surface to the MR coordinate space you want and then round the ['rr'] to the nearest 'mm' (since all FS MRIs are conformed to 1mm). This should give you the nearest voxel.

Though we may want to consider a lot more options in the future (maybe include a comment) for this. Like multiplying across the ['nearest'] field, or smoothing (even though I hate this, it is an MNE default) across the undecimated surface then pushing that "full" data out to the volume etc. (but don't worry about it for now)