I was able to boot from a SATA disk that was manually setup with a GPT-formatted table and a debootstrap'ed debian arm64 kernel. The GPT partitions included:
1. BIOS from sector 34-2047, no filesystem
2. ESP from 2048-409599 (which u-boot calls bootable even if the flag isn't set), vfat filesystem
3. XBOOTLDR from 409600-819199 with bootable flag set (0x4), ext4 FS without journal
4. linux swap
5. ARM64 root (gdisk type 8305) with bit 60 set (read-only), ext4 without journal
6. linux /var (gdisk type 8310), ext4, with /srv/ and /home/ bind-mounted from /var/local/
The bootscripts/extlinux configuration live in /boot, onto which is mounted partition 3 (XBOOTLDR) next to the actual kernel/initramdisk. It is necessary to mark this partition bootable in gdisk to have the u-boot distro-boot search it for extlinux/boot.scr.
While the espressobin doesn't require u-boot flashed on the boot media, this disk partition scheme would allow rockchip64 systems to boot if the BIOS partition is extended to somewhere around sector 32767 (as rockchip needs code at sector 64 and maybe 16384 and 24576).