Fortunately there was an easy fix. The nice people making the Pi have put large test-pads on the back side of the board. So I just grabbed an extra microSD adapter and soldered it to the back of the board.

And the PiZero boots up nicely again.
TOP LINE
1 - PP19
2 - PP15
3 - PP5
4 - PP8
5 - PP14
6 - PP5
7 - PP16
8 - PP17
LOWER LINE
9 - PP18