I agree, my first thought when I saw the pics was DSLD.I personally would not advocate breeding a mare with hind legs that straight. No matter how outstanding the stud's conformation is, the mare still contributes half the genes.
There would be no guarantee the foal would not have hind legs as straight or even straighter. If so, the foal would likely have stifle problems and most likely would not be able to jump. Horses with very straight hind legs are prone to locking stifles and/or lifelong pain issues. If your mare does not have pain issues, she is very lucky. Are you certain she does not have DSLD? In some of the photos her pasterns appear dropped. It is hereditary but I don't think they know yet how it is passed on exactly.
IF she was mine, get her checked for it prior to breeding.