I wrote a script that can be run under scapy that will test the list of hard-coded IP addresses that the Bitcoin client will attempt to use for bootstrapping wihen IRC is not responding. When I tested, I only got a response on 70 of the 230 addresses, the remaining did not answer within the 5 second response time I arbitrarily chose for the script.

The addresses in the script were copied from the pnSeed assignment in:
https://github.com/bitcoin/bitcoin/blob/master/net.cpp

Script:
https://gist.github.com/ce222620c49c5efddf4f

Usage:
$ sudo scapy -c test_bootstrap_backup_addresses.py

Results:
https://gist.github.com/e371524c09a71e1731ef