Businesses that are worried about mobiles being used in unauthorised places uses technology that might be useful to you.
I wrote an answer on the Academia StackExchange with some products but I think only AirPatrol (though I note on the site AirPatrol now says they don't collect IMEIs, check with your sales rep) and Libelium will be relevant. Some of these you can buy multiple sensors to work out which "zone" the mobile device is being used in.
These will typically list the IMEIs of broadcasting devices and as they do not interfere with them or broadcast on licensed spectrum themselves I believe are legal (but IANAL). Still I'd be surprised if they were illegal to operate given they sell them!