... die Wahrscheinlichkeit für das Eintreten des gewünschten Ereignisses ist im Freien bedeutet höher als 'indoors' . . .

... try : beim '(Ab)Hocken': Dictionary: 'hocken . . .

@Daddy: I am only getting paid for translating that crap, I didn't, like, wanted to get into all too much depth of it... ;op
But thanks nevertheless, mate.

It's a well-known German curse, but I'm not sure translating it word for word would make much sense. Perhaps limit yourself to just "May lightning strike you!"?

If you must:

... while you're sitting on the can!
... while you're ...ing.
... in the outhouse.

may lightning strike you on the loo

- provides a little alliteration and rhyme at least - but AFAIK loo is very BE?

May lighting fry your butt while you're sitting on the throne.

It's just me, but what a way to go: Lightning Struck on the Crapper

That would make headlines, and it's almost borderline enviable.

On-topic: May lightning strike your bowels during their movement.