Sample code for integration in a Xamarin Studio addin to implement swizzling of System.Reflection.Emit enabled core assemblies ("REPL-enabeled") for Xamarin.iOS. Refer to https://bugzilla.xamarin.com/show_bug.cgi?id=48239 and https://github.com/praeclarum/Continuous/issues/46 for more context.

ios-repl-swizzle.cs

// ios-repl-swizzle.cs
//
// Copyright 2016 Microsoft.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

class SampleAddin
{
    static SampleAddin ()
    {
        IdeApp.ProjectOperations.EndBuild += HandleEndBuild;
        // You might need this event if EndBuild doesn't work
        //IdeApp.ProjectOperations.BeforeStartProject += HandleBeforeStartProject;
    }

    static void HandleEndBuild (object sender, BuildEventArgs e)
    {
        var project = IdeApp.ProjectOperations.CurrentSelectedSolution?.StartupItem
            ?? IdeApp.ProjectOperations.CurrentSelectedBuildTarget)
            as DotNetProject;
        // Obviously you'll also want to verify that it's a Xamarin.iOS project
        if (project != null)
            Swizzle (project);
    }

    void Swizle (DotNetProject project)
    {
        var appdir = Path.ChangeExtension (
            project.GetOutputFileName (configSelector),
            "app");
        var appAssemblyDirs = GetAppBundleAssemblyDirectories (appdir).ToArray ();

        var mscorlib = project
            .AssemblyContext
            .GetAssemblies (project.TargetFramework)
            .FirstOrDefault (asm => asm.Name == "mscorlib");

        var targetFrameworkPath = TargetFrameworkPath = Path.Combine (
            Path.GetDirectoryName (mscorlib.Location),
            "repl");

        foreach (var asm in Directory.EnumerateFiles (targetFrameworkPath, "*.dll")) {
            foreach (var appAssemblyDir in appAssemblyDirs) {
                File.Copy (asm, appAssemblyDir.Combine (Path.GetFileName (asm)), true);
                var mdb = asm + ".mdb";
                if (File.Exists (mdb))
                    File.Copy (mdb, appAssemblyDir.Combine (Path.GetFileName (mdb)), true);
            }
        }
    }

    static IEnumerable<FilePath> GetAppBundleAssemblyDirectories (FilePath appdir)
    {
        foreach (var appAssemblyDir in new [] {
            appdir, appdir.Combine (".monotouch-32"), appdir.Combine (".monotouch-64") })
            if (File.Exists (appAssemblyDir.Combine ("mscorlib.dll")))
                yield return appAssemblyDir;
    }
}

Note that this is a rough idea of what the Inspector addin did at one time. You may have to tweak things, particularly around addin hooks. The bulk of this code should be extremely relevant for integration however.