This is a bad idea. In the way of background, I worked as an ops engineer (MCSE and CNE) in the early to mid 90s migrating from IPX to TCP/IP. Then as a developer from 1998. Then as a tech entrepreneur from 2003 onwards. I've created highly scalable and very popular applications and am currently the CEO of Defiant Inc which is a cybersecurity company that employs 40 and protects over 4 million websites.
Please don't do this.
1. You're using a labor intensive bandaid instead of solving the fundamental underlying issue. Apply this energy to IPv6 adoption rather than building a faster horse.
2. You're going to patch OS's, but the net is powered by a bunch of old unmaintained gear and reality will not match your expectations. Look at the number of vulnerable routers in developing countries for an example of unmaintained gear.
3. The problems around what you're doing are going to manifest in destructive ways. Primarily users of the new block being unreachable, and users of the new block being DDoS'd when they are reachable.
Furthermore, I don't think this brief draft fully explores the consequences of making this change. It would require real-world testing of a lot of gear in the wild, in combination with backbone router behavior, to fully understand the consequences of doing this. The proposal shifts the responsibility of doing that testing, or accepting the consequences, onto operational staff and teams, which is unfair and creates a lot of work and late nights for those teams.
I'd also point out there is a non-zero risk that the global intelligence community will take an interest in acquiring, or commandeering, some of this address space to capture the junk traffic it will receive once live.
This isn't unused address space. It is, and should remain, unusable address space.