The issue you're going to run into here is platform-specific representations of textures. You have to compile textures for the target platform so they can be loaded with the GPU.
Luckily, Protogame already supports doing this at runtime on desktop platforms (Windows, Mac, Linux). MonoGame won't out of the box (it doesn't have the asset infrastructure that Protogame does). But if you implement and bind the interfaces I mentioned in the other thread, it should all "just work".