Xamarin.iOS (MonoTouch) does not really depend on an specific Xcode, but some things will adjust themselves with the installed version.
You do not need to hack your currently installed Xcode (too many things can go wrong) but you can install several versions of Xcode side-by-side, e.g. I got 4.2, 4.3, 4.4 and 4.5 on my same Lion laptop. Just rename the one installed at the default location before installing a new one.
When you select an old Xcode (from Xamarin Studio, or older MonoDevelop releases, preferences) it should adjust to the right features. IOW once 4.4 is installed (and set as default) then ARMv6 should be available.