We don't really have any public documentation for the file format.
Feel free to post any specific questions here, or email us directly at
omniplan@omnigroup.com
Another approach (which the PM also may not like) would be to create the CSV file, then have an AppleScript launch OmniPlan, import the CSV file, and save it as an .omniplan file. This would probably be easier to get right, but would require that your web server have access to a mac with OmniPlan on it.