new feature: limit registration of blogs to specific domain

I am developing a school blog site and it would be helpful if we could allow user/subscriber registration to any domain but limit registration of users with blogs to specific domains. Would be even cooler if we could limit it to one level of the domain. in the U.S. many schools' domains appear like this:

schoolname.statename.k12.us

if we could limit registration of blogs to just any email address that contained *.*.k12.us this would be amazing.

thanks guys.