From my experience with using Blender as a level editor, here's some tips:
- None of the existing exporters will currently export custom properties of various objects. They are vital in supplying metadata so if you want that, you will have to make your own exporter.
- #blenderpython IRC channel (which can be found in the same server as other channels: http://www.blender.org/community/chat) should be able to help you with most of your problems (please do consult the documentation first though, it will help you find most variable names).
- If you want to export more than position data (meshes, for example), Blender's data requires lots of preprocessing so I highly recommend using at least one intermediate output stage.
- You can use F8 to reload scripts instantly. You can enable the console to find out why your script isn't registering the necessary menu items.
On second thought, if all you need is positions, why Blender? Is there anything bad about just making a "position viewer" app based on your game code (you could even load the level and instantly see it) and writing the level files yourself?