Updated for V1
I've been meaning on doing a bit of a writeup on this since I hadn't really found any tutorials on the web (as of this writing), but my laptop is going to make an unscheduled visit to Dell to get a new keyboard. As it is I had to plug in my desktop's keyboard just to type this up :-)
In any event, what we have here is an example of how you would write a content pipeline processor that has external shared references to other things like textures. The example code uses a filetype that I made up for the sample, a .tlf file. below you'll find an example .tlf file.
This is an arbitrary string
Textures/brickwall.png
The file can contain any number of textures (relative path), where the first line of the is simply an arbitrary string. Since the importer imports all of the referenced textures, you only need to add the .tlf file to your visual studio project for your content to be built and included in the build. Also, if two .tlf files share a reference to the same texture, the file will only be built once.
I'll be using this as a basis to build the content pipeline for the XNA version of the FlatRedBall 2.5d Engine which I am helping with as I have time :-) http://codecube.net/xna/ContentPipelineTest.zip