Otherwise if this thing has the right version of sonoff firmware, then I can use something like sonata to flash it OTA. But this thing must be a Sonoff clone, so there's no telling if Sonata would work. But I will at some point soon try it.
The other approach would be painful - you might be able to build some sort of contact jig that fits over the top and makes contact with the serial pins. I've done this before by spraying the unit very lightly with oil and then covering it in white glue to make a plastic mold - once it's set, you then push wires through at the pins