I have heard of a few custody court cases. One was a high profile one in which a woman appeared on MTV with her husband and boyfriend who lived with them in a V relationship. The woman's grandparents heard about ti and tried to sue for custody. The judge was going to make the woman give up her children unless the other guy moved out. He moved out and last I heard they were still fighting it. However, that was about 10 years ago. The judge was heavily critsized for pushing his personal views.
I believe that harm has to be proven rather than assumed for people living in alternative relationships. (A lot of this about from gay rights.) I believe I heard that there is a polyamory legal activist group. Maybe someone on this board knows more about it than I do.