I think thats a great idea. If it can be done using some kind of token so there is no need to enter your password into one site as previous sollutions have done then that would be great. ie, click "connect" and enter url of other plone site. Gives you a popup where you login and that results in an exchange of a single use key just for exporting. Then you control the whole thing from the site you are importing to, select content, filters etc.
Out of scope of this project I would image however 