I agree with every word you say Phil, EXCEPT that, as the baby is fine, trying to identify an antibody directed against a low-prevalence antigen is like hitting your head against a brick wall, over and over and over again! One has to remember that it is not just the specificities within the 701 series (assuming that it is not a novel specificity all together), but most of the "larger" Blood Group Systems also contain low-prevalence antigens (just look at the Diego Blood Group System, as an example); you could spend many "happy" hours testing the plasma, and still get nowhere.
So, the serologist in me says "YES", the pragmatist in me shouts "NO"!!!!!!!
The thing is, the next pregnancy, if there is one, should, without doubt, be closely monitored (probably by MCA Doppler, or something similar), but, if either an IUT or an exchange transfusion (or a top-up transfusion, come to that) is required, finding compatible blood would be easy, even without knowing the actual antibody specificity.