Theoretically it is quiet easy to fake that double-tap with software, but there might be some obstacles in practice.
The bootloader uses an unsigned long value in RAM memory to indicate if the double tap happend. The actual bootloader for the zero uses the following definitions.
// zero specific; but other boards use the same values AFAIK
// (in ...
Well after "working" on this all day, it turns out that the solution was to use the "Arduino as an ISP" solution as described on the arduino.cc website.
Why the USB-ISP solution did not work is not immediately obvious as I have used it successfully before - just not with this board!
FWIW, the blinking sequence I observed when burning the ...