You can dock the Zotero extension to the existing UI if you don't want it to float. We'd like to support the notebookbar and we have had some community contributions in that direction, but the end-result looks considerably worse and more confusing than the standard interface toolbar, so we have elected to not even include it. Document Foundation doesn't seem to be putting much effort into the notebook bar recently, including things like making automatic and good looking extension UIs, like Word did when they transitioned, so the people you should consider encouraging are LibreOffice developers.