Is it possible to determine the model of an iOS device by its MAC address?
No - it's not a good way to get that data.

I would have a look at some of the many MDM software packages to get your population data so you can plan. JAMF has a great reputation in that space for low cost of entry and high performing software.

Also - you could make a quick little app that simply reported in some basic stats. Your users will be clamoring for potential apps and those willing to run that app will feed you with the data needed.

You will find that making the app forces you to restrict many of the new features and in reality - you'll have to actively work to get 3.x support in a project you start today using the current Xcode 4. 4.0 will be easy and only the new 5.0 features will get you into having to test the levels and have two sets of code for different generations of device. 

Your development will drive what devices people use and not the other way around. It's almost always cheaper to just upgrade devices than pay developers to keep the older code tested and running.

There are lots of great charts out showing what some devs are seeing for how fast people upgrade to the latest iOS versions.