Do you need to tape and 'mud' cement board corners in dry settings?
The thinset and mesh tape are to strengthen the seams. If you are going to tile over the cement board, you should tape the seams.

This should not be that difficult. Pack in thinset, lay on the tape, and flat-knife to get it set in. In the corners, use a corner knife.

If you just want to leave the corners untreated that will probably be fine, because that seam won't split the center of a tile.