[hzfoblrm] Making announcing to a Freenet location expensive

The Freenet threat model states that darknet is more secure than opennet because "compared to opennet, where you can just announce to a location, it's very expensive."

Can it be made more expensive to announce to an opennet location?  Perhaps some sort of Hashcash-like or PBKDF hash computation.

How often will it need to be checked?  We only want to make it expensive on the attacker, not every user.