That said, with flat-ground ways, parts could sit flat, or on top of parallels or some other positioning jig. The angle wouldn't be well defined, but a plane could; this might be adequate for some operations.
Mostly, I doubt the clamping force is all that great, and you wouldn't want to put an end mill near the thing. Or you're taking very light cuts, which will take forever for bulk removal -- but would be fine for engraving purposes, again.
With a CNC indicator to measure part position and orientation, I suppose one could set up without too much bother, and engrave/shape completely arbitrary parts. (Is that something CNCs can usually do? Position offset obviously, but multiple rotations?)
Tim