I found a morge aggressive way to bypass the WDT interrupted SLEEP_MODE_PWR_DOWN. Since my pro mini only wakes up for a few µs, the time was to short, to exactly release the reset button while the programmer was trying to upload the sketch.
I ended witch "scratching with a cable" over the entire lower right site of the atmega328p pins. This way I've done some short circuits betweend pin 3(GND) and 4(VCC), pin 4(VCC) and 5(GND), and pin 5(GND) and 6(GND). The Atmega have to do a full reboot (and mybe some other stuff), wich seems to take longer than "just" doing the reset.
This is not a goot Solution, but it worked for me.
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\ / <- over the entire side
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