I am also a little late, I was on holiday, well that's my excuse.
Anyway, I had the same problem. The way I solved it was to write a class that extends ContentManager.
From within this you can still use pre-built content, but if the content is not there you can fall back to manual loading.
At the moment I am handling my custom content types this way instead of using a new content importer, and I handle textures by loading them into memory and then creating a Texture2D from the MemoryStream.
The reason I do this is two fold.
It actual fact it is very easy to do and although slower than using pre-built assets works incredibly well.
If you have problems send me a line.