From my experience, usually the content (xnb files) are compiled from some other intermediate format. (for images its bitmaps or .pngs, or what have you) I'm guessing you have some kind of xml format.

To get to that xml format, usually you write some kind of serializing code or use a serializer class (like XmlSerializer or IntermediateSerializer or some kind of serializing/persistence framework)

So you could use your TileData class in your editor, save the map as an xml file and you could run your visual studio build or set up some MSBuild stuff in your editor.

All in all, I think your looking at this the wrong way and that you shouldn't compile to xnb directly. I'm not an expert, but I think your a tad confused with the workflow, at least from my understanding.

My Understanding in Digest Format : Runtime Type (in editor) -> Save to intermediate format (xml or whatever) -> build later at design time (this is where that Content class comes in)

This is also a bit easier to debug because xnb files are binary while xml is filled with strings, far easier to look at and understand. Maybe I just didn't understand your question/situation.

Just in case I'm completely off: You could use the runtime type and when you save it you could instantiate a bunch of the content types to save with?