I wouldn't bother, to be honest.
Apart from the fact that the Lutheran antigens vary in strength of expression, making it difficult to ensure that the recorded titres would "match up" one to another, but the expression of the Lutheran antigens on foetal and cord erythrocytes is known to be weak. On top of that, of course, there is the problem of finding a regular source of adult erythrocytes with heterozygous expression.
In addition, anti-Lua and anti-Lub can be either IgG or IgM but are more commonly IgM. It might be worth your while treating the maternal plasma/serum with a reducing agent such as 0.01M dithiothreitol, 2-mercaptoethanol or ZZAP to see how much, if any, IgG is present.
Even if the antibodies are IgG, they are thought to be adsorbed on to foetal Lutheran glycoprotein on the placental tissue.
Lastly, as you so rightly say, clinically significant HDFN caused by anti-Lub is incredibly rare, and so, all in all, you could be giving yourself an awful lot of work for very little return. If you do decide to test the maternal plasma/serum with reducing agent, and you find that there is an element of IgG present, it might be worthwhile just performing a titre once, in order to see that you have not got one of these incredibly rare examples that might cause clinically significant HDFN, and, as lone as the titre isn't massive. I would rest easy.
If you want, I can cite references to back up what I have written above, but I haven't done so straightaway, as actually finding some of these papers to read is equally hard work!!!!!!!!!!
I hope that helps.