Never come across that cross although im sure it will have been done by somebody, possibly as an accident. The ewe lambs might be handy for breeding as sort of a slightly upgraded 3/4 hill ewe but I can't imagine they would grade out very well as neither herdwicks or mules do. Mules are bred to enhance the abilities of the hill ewe and as such the ewe lambs are very sought after and put to a terminal sire produce a good lamb, their male lambs are more of a byproduct. One thing I would say is herdwick ewe lambs in my experience are not usually well enough grown to be tupped and it really sets them back. We do tup a lot of ewe lambs of many different breeds but herdwicks are one of the few breeds we just don't do it on any more.