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By far the most flexible approach with all-around utility appears to be exposing unAPI in Fedora.
One noteworthy detail: Zotero doesn't appear to parse any other format than mods, at least when using unAPI. In my server access logs, after the request for the object (e.g. http://example.org:8080/fedora/get/demo:1), the next request is for the mods format (e.g., http://example.org:8080/unapi/fedora?id=info:fedora/demo:1&format=mods). I don't see any requests for the various formats that might be available for that resource (e.g. http://example.org:8080/unapi/fedora?id=info:fedora/demo:1). This is using Zotero 1.0.7, I haven't tried the 1.5 preview release.
Once I added mods to my Fedora objects, browsing to the object's profile view yielded the little blue icon in my browser's location bar, indicating that Zotero could grok my Fedora object.
DSpace Integration
As a proof of concept, I also implemented an OAI-PMH resolver, designed to provide unAPI services for any application that exposes OAI-PMH. As DSpace provides OAI-PMH services, providing Zotero support only involved the additional steps of enabling the MODS crosswalk and adding the unAPI microformat identifier and unAPI service link to display-item.jsp.
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