The very small bits can be sharpened on a setup that includes a pin chuck with a Tee shaped body that includes a couple of small rollers at 180 degrees. Included in the set is a loup to see the bit tip for setting, a setting gage for tip angle (exposure), a fine six inch long stone (approx.280 or finer) and the chuck/roller with additional collets. These sets are probably still available commercially with a bit of searching. Being miserly, I enjoy not having to throw away dull micro bits, they sharpen surprisingly well, sometimes better than new.
This pin chuck device could well be fabricated and if anyone is interested, I will gladly include a photo or two...
Bill C.