I finally got this to work. In short, I began with a branch of the Samsung Artik 710 vesion of U-boot and made a variety of modifications. There was a bug in the way U-boot was setting up the MMU that prevented it from correctly accessing the second 512M of ram (each page table entry covers 512M). T...
Yep, the Artik 710 resources can be helpful and informative given that the Artik 710 also uses the s5p6818.
I've also spent some time looking for the sources for fip-loader.img and fip-secure.img. No luck yet.
It does look like there is an Artik repository on Github with sources for bl1-mmcboot.bin.