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Although the OAI-PMH resolver would also work with Fedora's OAI provider, the Fedora resolver allows for more flexibility. The Fedora resolver is not limited to the formats exposed by OAI. If Zotero integration is the only use case, this additional flexibility isn't important. It shouldn't be difficult to write a DSpace-specific resolver as well. I'm just not familiar enough with DSpace's API to do it myself.
Future Work
This work is part of a general Zotero integration effort. Making Fedora content Zotero-friendly is just one side of the coin. On the reverse is using Zotero as a client to Fedora. If people are already using Firefox & Zotero to browse, cite and archive resources, it's just a small leap to imagine Zotero as a client that enables users to save those resources (with proper metadata, no less) to a Fedora-based repository, and to share, preserve and re-use that content in the context of a Fedora repository. And of course others will come and use Zotero to cite, annotate and share this new Fedora resource anew.
The Zotero 1.5 preview release showcases initial support for saving content to a remote server (in contrast to the local SQLite database used in 1.0.x). Unfortunately (from my perspective), it looks like Zotero's remote sync feature requires WebDAV support and I haven't heard of any plans to support other protocols (AtomPub, anyone?). I'm wary of the effort that implementing WebDAV in Fedora would require.