There was an article in HSM or Machinist Workshop, within the last year or so, on using a slitting saw to make very thin grooves like that. It can be mounted on a small spindle and motorized to spread the cutting action over all the available teeth. That would give you considerable depth of cut, but if you need grooves on a long part you might need to make a special arbor, supported on both ends to get clearance.
"In theory there’s no difference between theory and practice. In practice there’s a lot of difference.” Yogi Berra