It would be an interesting challenge to write a macro that converts from Pages to ODF. I do not even know which scripting language to use, but I am sure it is possible somehow - probably mixing two languages.
Imagine a droplet on your desktop. You drop your Pages document on it; it opens in Pages; it is saved as Word; the Word doc is opened in OpenOffice or NeoOffice, and then saved as ODF.
And then another droplet for the way back.
Personally I do not have the time (or skills) to create such a thing. Besides it is not certain that the end result after two conversions would be usable.
However, if someone is curious and feels like taking up the challenge, please, go ahead.