We have a number of Apple iOS 5.1.1 corporate-owned, Wifi-only devices with internal applications that aren't ready for iOS 6. Is it possible to prevent a user from upgrading their device to iOS 6? The only solution that I've found so far is to block network access to Apple servers...but I'm not sure I have a complete list of hostnames.
I'm afraid not sorry.
That said the vast majority of iOS apps are either 100% compatible between v5 and v6 or need a simple recompile with the XCode 4.5 to be ready, and dev releases of v6 have been available for 4-months+. So it's a shame that your devs didn't put the effort into fixing the root-issue, it would've save you having to go out of your way to find a workaround.
You may have some luck with OS X Mountain Lion Server as it does provide some policy management over iOS devices with the configuration profiles feature. I have not tried it, but for $20 USD from the App Store, it would certainly seem to be worth looking into it if you have a large number of corporate iPhones to manage.
There were also rumours of RIM adding iPhone, Android, Windows smart phone support in BES, but I'm not sure whether that was ever rolled out or not and what version of BES (BPS, etc.) that applied to, if any.