It's like the Beagle Brothers paying a two-minute ad announcing they're going to rob Wall Street at 3PM with tommy guns, only done by someone who somehow couldn't find a better method of getting laid.
Except not at all. For this exhibit, in order for him to actually RAPE anyone, they have to GO THROUGH THE GOD DAMN TUNNEL.
It's more like the whatever brothers announcing that they're going to murder someone, but only if they let them. The act of WALKING THROUGH A LONG TUNNEL THAT GETS SMALLER AS YOU GO IN, while KNOWING DAMN WELL THAT YOU WILL BE SEXUALLY ASSAULTED, requires volition. A will! A desire to go down the tunnel, and get raped. It's not even a semantics argument.
Now, if someone was forced down the tunnel by someone else, the question is more interesting: Who is the rapist, the one who committed the assault or the one who forced the victim down the tunnel?
EDIT: I realize now that I did not make myself clear. I do not think he should be arrested for the announcement, or the exhibit itself. If he actually goes through with it and rapes someone, however, him getting arrested and taken to court is 100% reasonable, and the artist damn well be prepared for a lengthy court battle. The argument of whether or not it is rape is something that should be decided in court anyway.
EDIT 2: On an even more inappropriate note, that court case would be the first in recent memory where the "They was ASKING for it" defense could be taken even remotely seriously.