This archive post noted that a reboot to the install environment, the eMMC was seen.
http://openbsd-archive.7691.n7.nabbl...-td352781.html
I found this presentation from Samsung on the structure of eMMC. Slide 3of21 shows the RPMB partition may be sector 0.
https://www.slideshare.net/linaroorg/intro-to-emmc
FreeBSD 12amd64 Release does recognize the eMMC drive
Code:
mmc0: <MMC/SD bus> on sdhci_pci0
mmcsd0: 31GB <MMCHC DF4032 0.1 SN F626303D MFG 04/2018 by 69 0x0000> at mmc0 200.0MHz/8bit/8192-block
mmcsd0boot0: 4MB partion 1 at mmcsd0
mmcsd0boot1: 4MB partion 2 at mmcsd0
mmcsd0rpmb: 4MB partion 3 at mmcsd0
I was also able to print the partition table using FreeBSD's fdisk.