I found it most suitable to install OpenWrt onto an interior SSD partition from a running TOS instance:
- downloaded openwrt-19.07.3-mvebu-cortexa9-cznic_turris-omnia-sysupgrade.img.gz
- unzipped it
- mounted the two partitions from the image with
partx
- copied the content from the two partitions onto the SSD's (single) destination partition
- edited the boot.scr to add automatic switching between copper and SFP
- adjusted the u-boot boot args
OpenWrt now happily running from SSD.