http://www.codeproject.com/KB/aspnet/ASP2UserControlLibrary.aspx
Seems like a reasonable idea, as he points out.. LoadControl relies on finding the actual ascx file.. but if you embed the ascx into the assembly, and then create a virtual path provider to intercept a specialized url.. you can use LoadControl to load against that url, which in turn streams the ascx out for LoadControl to use..
Also, the comments at the bottom have another suggestion for a small framework on code plex that can handle this..