I would do a structured informal rush -- essentially, one round of open houses required for all rushees, and the rest of the rounds invite-only with rushees picking which houses they want to go back to. For pref round, girls could go back to a max of three houses and quota would be determined as usual.
This is essentially how my school ran spring recruitment (except that girls could go to as many pref parties as they wanted -- although few went to more than two -- and could receive multiple bids), and I liked it a lot. I found it combined the strengths of COB with the strengths of formal rush. I might be biased since it was the type of recruitment that my chapter always excelled at, too.
