Visual Studio does not have a form-builder for IronRuby as it does for C#, etc. An alternative is SharpDevelop, which is much like Visual Studio, except it supports form building in the editor. Either way, this isn't what I meant to do with IronRuby. I meant that you create your application in C#, and then use IronRuby's interface to do some things that need done in Ruby, such as reading Marshal format, which is really all you need. Just have IronRuby load the RPG module into memory, and from there you have the ability to create Ruby objects that can interact with C# ones. As for the code completion, I really don't know about that, mine seems to work fine, so I can't be of much help there.
As for the Sprite class, there are native libraries in .NET that you can get the same result from. The Sprite class in RMXP is written in C and interfaced with Ruby, so either way you're really not going to be able to use it without creating your own or using an existing library. I don't really know how much need of the Sprite class you need in an editor anyways.