I'm trying to write a Python script to convert between those formats mentioned above, and I haven't been able to find a suitable library that is capable of doing this. Suggestions?
The .pages to .ppt part I'm not sure about (I'd assume you'd want a keynote file for doing that). However with the .pages to .doc part you might want to look into the PyObjC Scripting Bridge support interfacing with Pages. There is a guide which looks at doing so with pages, changing the .rtf references to .pages and the "SLDocumentTypeRichText" to "Microsoft Word 97 - 2004 document" for doc support.