Consider the demands (resource wise) for even a small DNSBL. These would pretty much equal if not be lower than that of a hand full of users utilizing the IP Board Spam Registration protection.
It's dooable, but would require sacrifices or offloading it to customer servers.. I'd be more than happy to allow Inivison to run a script on my server to act as a p2p but than again, that would also create problems due to malicious site owners ;)
In regards to the ban thing, Hide banned members from the public user lists.. Problem solved