It's not much help, but it's something... if you have Visual Studio handy, you can actually re-purpose its designer to build MonoGame UIs for you. Just write controls that ultimately generate MonoGame code for you. Then you can at least drop stuff on a canvas and drag it around, as well as easily update positions, anchors, and what have you.
I got tired of trying to place UI elements programatically, nor did I want to write an editor, so I went with the mishmash method and it actually worked out rather well for me.
Also, I hear you >.<