I've had (and have) the same problem. My institution has a WPA2 Enterprise wi-fi network, which means that the AppleTV cannot easily join it. I've tried installing a corporate certificate, but no way.
As explained in other messages, AppleTV mirroring can be done with an ad hoc network. This can come from (a) a computer sharing its Internet connection via wi-fi; (b) an Airport Express or another Access Point device; a mifi device which distributes wifi from a 3G connection, or... (d) the simplest one, a smartphone (or the device itself) sharing 3G via wifi (personal hotspot).
Now, iPhones have personal hotspot; so do the latest iPad models, right? So nothing else than the device itself would be needed.
If Internet is not needed for the presentation, perhaps the device wouldn't even need to have a data plan. Or, if the phone has it, it need not to be used at all. I have tried this with an Android phone with a data plan: I created a fake name (for example, "appletv") APN (the address the internet service needs to get the data) which of could prevented the phone from reaching the Internet; but the personal hotspot, or personal access point, or however it is called, works anyway, for free.
So, this would probably be the least obtrusive way to have a local wifi network IF the device to be mirrored can't do it.