If your points are symmetrical and you construct a plane from each set of points, your midplane should work fine. Unless the edge faces are so rough. you could also set the part up with a flat piece of stock touching each face as a datum simulator. Something mechanical rather than code.
Can you napkin sketch a visual of the part, and how the dimension is on the drawing? It's kinda difficult to help, when we have no idea of the shape of the part.
I would think if you could level to an axis line (presuming the part is some type of tube, since you said it's hollow).
you can use a mid plane and offset the angle . what are the datums? Scan around that body and construct from that. Is any part of it supposed to be cylindrical or circular.