not sure if this would be considered a domain hack, but it flows well. first thing thing that came to mind was all those "get your free ___" here sites, where people can get their own ipods and whatnot by signing up for things, getting referrals, etc. if you can develop it to something like that, whereby people can get a free .net domain, it would be pretty cool.
success will depend on hype, word-of-mouth, and your promotion. revenue will depend on how much work you want to put those poor people through to get their .nets. high $xxx+ if you can really get it going.