So, which of the standard bibliographic databases I use everyday are compatible with Zotero?
Answer: None of them. MLA, WorldCat, JSTOR, and the rest work fine for secondary literature, but ISTC, GW, VDiB, ZVDD do not know about Zotero. Or rather, Zotero doesn't know about them. Someone needs to write a web-scraping plugin for each site, and apparently no techie early modernist has done it yet. I've worked on similar projects before, but I'm not sure when I'll get around to looking at the inner workings of Zotero plugins. For now, Zotero will have to prove itself as a citation manager.