JSON is no replacement for XML
Don't waste effort supporting a complex XML standard from your native database. Instead, simply, and automatically generate an XML packet using existing nomenclature already established in the database. This means you will be able to develop minamalistic, simple, understandable, and scalable XML.
This G3 XML packet can then be consumed directly, or via a user-selected XSLT stylesheet. XSLT can translate the G3 vocabulary into any of the XML standards out there - including Atom, OPML, RDF/XML, and various RSS versions. In fact, supporting each of those is as simple as an XSLT file for each. Offloading the maintenance of the XSLT is a good way for the core dev team to delegate the work away in a Separation of Concerns design pattern. I recommend getting help from http://www.mulberrytech.com/xsl/xsl-list/
Bear in mind that there are a HEAP of slideshow presentation technologies out there that use XML files as input. They can provide their own XSLT files to go from native G3 XML to their own private vocabulary. Many of these technologies also support various XML standards (OPML/RSS mostly).
I have approached the Miro team about the prospect of "image stacks" as channel items. Right now they only support video and audio. It would be great to periodically publish photo sets (albums) and content channels for readers like Miro/iTunes and various other readers.
On the issue of "connectedness", REST is inappropriate as it is very much in alignment with the HTTP paradigm which is inherently connectionless. However SOAP is appropriate because you can return objects which are elements in linked lists (which is essentially what you have described in your requirements). I supposed you could return these as JSON objects.
references: http://www.mulberrytech.com/xsl/xsl-list/ http://xao-php.sf.net http://www.monoslideshow.com/forum/discussion/2096/php-script-for-automated-xml-config/ http://www.flashnifties.com/support/viewtopic.php?id=2040 http://wiki.slideshowpro.net/SSPsa/EX-PhpXmlBuilder