It's only a /27, they were quite affordable a year or ten ago. A full /24 went for about $2500 back in 2015 when this was made and you can subdivide that to 8 customers who all get 30 usable addresses. Bit expensive for a joke, but not unmaintainably so.
It's silly that ISPs have messed up their IPv6 deployment so badly that there's a "shortage" of IPv4 addresses now. Of course, IPv4 was never going to be enough; there are too many people on earth.
I wonder whether you could do this with a lower number of IPs, just repeating them in the traceroute reply. You might not be able to build a real network like that, but if you're manually replying to ICMP with a raw socket, this should be possible, I doubt the client tries to de-duplicate based on addresses.
It's silly that ISPs have messed up their IPv6 deployment so badly that there's a "shortage" of IPv4 addresses now. Of course, IPv4 was never going to be enough; there are too many people on earth.