Good question. Someone sent me the link to a draft Pages document, which I could not edit on the Web. I downloaded the document onto my iPad and edited that copy using the Pages app. I emailed the link to the revised document to the original author. Using an iPad, that person could only download my version of the document, edit it on her iPad, and print the final result.
Following your suggestion, the original author opened my email on a Mac AirBook, clicked the link, and was then able to make her revisions using Safari. After she closed the document tab in Safari (I have no idea whether that step was necessary), I returned to the still-running Pages app on my iPad. After a delay of a second or two, my document acquired the original author's new revisions. When I modified the document once again on my iPad, the original author was able to follow the link in my original email to get back to the shared document in Safari, where she saw my latest changes.
So, going from an iDevice to a Mac via a shared iCloud link works, relying on Apple's Web iWork tools. If you want to go the other way, the recipient of the Mac email has to download the Pages document onto his iPad and send a link to the iPad copy back to the original author. Then both can collaborate using the shared iPad copy. Thanks for the idea.