I don't think we'll see socket SP3 Epyc on mATX for a few reasons. The chip itself is huge and you'd have to throw away a lot of PCIe lanes and DIMM slots that couldn't fit on the motherboard. Supermicro's ATX boards already halve the the number of DIMMs of their proprietary form factors or Gigabyte's eATX boards hold.
*Edited to clarify I meant socket SP3 boards rather than embedded.
