Hello David,
The IA was also one of the original supporter of the OPDS specification. I'm not aware of any OPDS catalog from DPLA (they seem to have their own set of APIs) but it would also be very useful.
There's really no rule on how one can organize an OPDS catalog, but if you want to make it useful for a human to browse (and not just a bot), dividing the catalog into multiple sections, providing a search option and using facets are all good to have.
I'm not aware of plugins for Omeka, Greenstone or DSpace but since some of them already have an Atom output, it should be minimal work. The only real requirement though, would be to make sure that these library tools systematically have a publication to download associated to each item (ideally in EPUB).
Best regards,
Hadrien