Subject pretty much says it all. How can I write arbitrary mesh data to file from Maya? (say I just want to export position info)

1) Write a python script to export the data.
2) Write a plugin to export the data.
3) Write a tool to open the maya file, get the data you need, and re-export the data.

We use option 3 at work. They are all close to the same, you just need to know the maya API for them all. The behavior is about the same. You iterate over the DAG, find the triangle meshes, and then iterate over the triangles/normals/verts and export them however you need.

For instance, there is a simplistic view of it presented here on GameDev.net. I suggest exploring the Maya API on their site for more information on how to do the more complicated iteration.