I've tried the following export/option combinations:
File > Export Selection (with desired object selected):
Output file per frame ON/OFF
Export selection output > Renderable scene/Scene fragment
The manual has the following to say on the subject of File > Export All, Export Selection (mental ray):
Export Selection Output
If objects do not change, reprocessing before exporting to .mi is not required, and the export process is significantly shortened as a result.
This is the default.
Maya exports all scene entities (lights, cameras, shaders, globals, and so on) that are necessary to render the selected geometry. The resulting scene will be renderable using the mental ray standalone.
Maya exports only the nodes that are selected. This mode can be used to export particular lights, cameras, shaders, or geometry. The resulting .mi file will most likely not be directly renderable, so it is called a Scene Fragment.
So with the Renderable Scene option, i get the object and it's instance in both file-per-frame and incremental files, but the file-per-frame shows no changes from file to file, and the incremental file has no incremental statements regarding the selection.
With the scene fragment option, the incremental file has neither object nor instance - it's basically an empty file. The file-per-frame files look the same - empty.
Anybody had a similar problem?