This phenomenon illustrates very clearly why it's so important to have inbound access lists protecting the router's own IP addresses on all edge interfaces.
If you want to stress-test the router's forwarding functionality, you could use the host route to the null0 interface; all packets sent to that IP address will be CEF-switched, so the only impact of the UDP flood to the unreachable IP address will be the increased interrupt CPU load. I was able to increase the interrupt CPU load to close to 50% on a 2800 router using a virtual PC with a Fast Ethernet interface.
And just in case you need it, here is the configuration from my test router. All packets sent to 10.0.0.22 are CEF-switched and dropped (the CPU load from the IP input process is negligible).
interface FastEthernet 0/0
ip address 10.0.0.1 255.255.255.0
ip route 10.0.0.22 255.255.255.255 null 0