Well, the first thing to say is that red cells CANNOT be either homozygous or heterozygous (or, come to that, hemizygous). These terms apply ONLY to genes, and red cells do not contain a nucleus. The antigens can only be described as, at best, "homozygous", "heterozygous" or "hemizygous" expression, or, alternatively, "double" or "single dose" expression.
Then, it HAS to be accepted that, unless the maternal antibody is an autoantibody, it must be an alloantibody (or, possibly, an isoantibody), which means that to mimic the state of the foetal red cells, the red cells used to titrate the antibody MUST have a "single dose" expression. However, that in itself presupposes that the foetal red cell antigens are all expressed at the same time, which we know is untrue (just look at the A, B and H antigens as an obvious example, but also the Kell antigens that are expressed much earlier than are the Rh antigens) or are ONLY expressed on foetal red cells, as opposed to other tissues (such as on the placental cells, which have, in some cases, been proved to adsorb the maternal antibodies).
Then, there is the fact that not all antibodies can be detected by all techniques. This is why Reference Laboratories SHOULD have more than one technology available (and their workers should be provably competent in these techniques. However, even then, not all techniques can predict the severity or otherwise of HDFN. For example, antibodies within the Indian Blood Group System always show that they can cause severe HDFN by certain techniques, such as MMA, but they don't! There is also the fact that the immunoglobulins may be IgM, IgA, IgG1, IgG2, IgG3 and IgG4 (to mention just a few), and I have yet to come across, or read about, an IgG4 immunoglobulin causing HDFN.
So, my answer is that there is a HUGE amount of knowledge known about the various antibody specificities, their titres, the expression of their cognate antigen, etc, etc, that there CANNOT be a single answer to your excellent question, but that the best thing that can be done is to read around the subject - and read around the subject from every source available - not just from a single country.
OKAY THEN, RIP ME APART!!!!!!!!!!!