Meanwhile I’ve successfully tried the same with a temporary mSATA SSD, except that the boot process moans a bit about not being able to read from the degraded, internal eMMC. Otherwise it seems to boot and work as expected, but not only me would prefer booting from USB.
I’ve found a thread from 12/2018, where some else seems to have the same problem.
That leads me to the conclusion that the official documentation is misleading by narrowing the use of USB to a not further mentioned “mini PCIe to USB 2.0” adapter and U-Boot/USB-controller don’t support it (yet?).