Thanks for the response! I'm actually looking specifically for a way to load models at runtime from unprocessed model files (particularly .X). I've already got them loading via the pipeline tool and that works fine, but to help make my game more easily moddable I want it to be able to load user-supplied models that haven't gone through the pipeline.
I achieved this in XNA 3.1 by loading .X format models via SlimDX and using reflection to stuff the SlimDX Model data into an XNA Model container, which worked fine. I could do the same again in MonoGame using SlimDX or SharpDX and it'll probably work, but it's a bit of a hack and I'm hoping there's a better way.