I just have to support this one - it takes me back soooo many years ;-)
Unfortunately, "bunk-up" also refers to, amongst other things, having sex, and the only dictionary (well, sort of) entry I could find was this one:
For the English side:http://www.allwords.com/word-bunk-up.html
A push or lifting action from below, to help someone climb up on to something.
Etymology: Early 20c: an alteration from the original dialect sense of bunt to push or butt.
I also found this (OK, not a great source, I agree, but I'm desperate *g*):
bunk up n. 1. Bonk; shag, As in: 'Darling, you look beautiful tonight. Any chance of a bunk up?" 2. Assistance climbing, as in: "Shit. I'm so pissed I can't get onto the bed. Giz a bunk up will ya?"http://www.milkinfirst.com/dictionary/b.htm
So I don't have much to offer in support, but I support the English side anyway (and, in my experience, Räuberleiter is a good translation). I don't suppose that that justifies an entry in LEO, but maybe it will help somebody who actually searches the archive after searching the regular dictionary entries :-)
Unless, of course, wibi does eventually come up with the goods...!