I  just put the bridge pu from my Schecter C1+ in the neck position and put in a
push-pull pot to split the coil according to the schematic on this website
(except I switched the black and the green wires, cause on my guitar the green
wire was the hot one, the black was soldered to ground). Everything works OK,
only: I get the full humbucking sound when the knob is pulled out, and the single
coil when the knob is pushed in. I would like it the other way around (series
when the knob is pushed IN). How do I switch that around?

On the switch portion of the pot, leave the wire that goes to the middle tab
alone, then put the other wire from the pickup on the other tab that is not
used.  This is on the switch portion , not the potentiometer portion