Heteromaternal Superfecundation - Any known cases?
This would be termed superfetation rather than superfecundation. While this extremely rarely happens in humans, superfetation does occur naturally in some mammalian species. These pregnancies occur when the mother ovulates after she is pregnant.

Superfetation consisting of a natural pregnancy and implanted embryo has been done in cattle. In this case, 7-day embryos were successfully transferred to 14-day pregnant cows to produce superfetation. See Joel Carter's dissertation Superfetation in Beef Cattle (2002).

There have also been successful cases of superfetation in cows that were first implanted with a 7-day embryo, and then followed by natural service. See WH Hall, Bovine superfetation by natural conception secondary to an embryo transfer pregnancy, Cornell Vet. 1987 Jul;77(3):282-3.

An excellent summary of the existing literature on superfetation in many species is K Roellig et al., The concept of superfetation: a critical review on a ‘myth’ in mammalian reproduction, Biol Rev Camb Philos Soc. 2011 Feb;86(1):77-95.