I think better approach for "ghost keys" would be requiring X amount of crypto to be sent to 0x0 (burning). Current implementation (requiring donation to freenet) basically gives freenet foundation infinite reputation (including any other potential project that would accept ghost keys as identity), kinda breaking the decentralization aspect
Ghost keys will ultimately be just one of a menu of options for bootstrapping reputation in a decentralized reputation system. They have the advantage of simplicity, anonymity, and helping to fund the project, but as you correctly point out - they are centralized. A cryptocurrency-based solution like you suggest will undoubtedly be one of a menu of reputation bootstrapping options that will develop over time.
Don't get me wrong, this is awesome. I think it is built on a subtlety bad premise. I think it is time to start build organizational nomic games on this sort of contract system, literal organization governance, for systems like this to thrive.