Not dared try it myself. UKC has occasional posts of woe from people who've had ropes and / or harnesses confiscated, seemingly in case they try to tie-up other passengers! That said, other posters on those threads have said they got away with it; very much a matter of the caprice of the security / baggage staff. If my weight allowance is marginal, I usually just put more innocuous stuff in my hand luggage - clothes, a pair of rock shoes, any books, belay specs etc