I understand your problem, but is it really that bad to load the whole tilemap? I mean loading it is not the problem right, but you dont have to render them all. What about if you just load the whole tilemap data into memory but only draw the tiles near the player. Everyone today has enough memory to store the tilemap data in memory, but what you draw is what makes the difference.
If you still want to just load a part of it, there is probably no way to do this with default ContentManager. Either you implement your own ContentManager which makes it possible to read files only partially, or you modify your tile map format into the chunks, create a custom importer/reader/writer/processor which reads the chunks and you create a chunkmanager combining those...
I think of it like that if your tile maps name is for example "Town":
That would be a 2 chunk tile map. Now lets say your tilemap is 64 by 32, each chunk has 1024 tiles.
I think you get the point.
Edit: The thing is when you want to load chunks when game is running you would need asynchronous content loading, I read here on forums already that its theoretically no problem, but it is also not designed for it.