Blogbridge have enabled users to combine their Reading Lists into one big OPML file…I wonder if you can select which Reading Lists you would like to include.
Anyway what this seems to do is organise your Reading Lists into folders within the one OPML.
An alternative could be for each BlogBridge user to have a root OPML URL (a mother OPML for all Reading Lists), where each Reading List could be an OPML inclusion in the mother OPML.
You could do this using OPML Workstation…just make an OPML, and each item is the OPML URL of each of your BlogBridge Reading Lists.
The difference here is that a flat Reading List, where each folder represents the contents of one Reading List, only has one OPML for the whole lot.
Whereas if each Reading List is included as an OPML inclusion, then someone grazing the list can choose to subscribe to the OPML of a single Reading List, or subscribe to the OPML of the mother Reading List.