I think you missed the point - if people show up at $PROXY expect nice stuff but see junk, then they won't move over to $REAL and instead blame $REAL.
E.g. you'd like some way to redirect people from $PROXY site to $REAL site, and disgusting content on $PROXY won't do that - it'll reflect poorly on $REAL
It's a proxy, so there's no "crawler". It's just an agent relaying to the user. Passing something to this proxy agent just passes it directly to the user.
Love the idea.