It occurs to me now that it should be possible to synchronize with almost any OPML file (even through HTML and ignore the FTP layer), as long as the define a few of the correct elements. The next version of the proposed standard will make this clear and adjust some of the MUSTs to make sure this is possible.

Once this is so, the story becomes quite simple:

• You can synchronize against arbitrary OPML files
• OPML files can define a few extra elements to make synchronization simpler
• One can define a common repository using FTP -- or other -- to make a synchronization point