I know it is not the answer you are searching for, but it might help someone to provide a solution.
Using GLADE, I managed to add button “Insert citation”, to existing Reference tab in Tabbed style… However, I could not add a separate TAB for Mendeley only. Here are the titles, commands, and icons used by Mendeley. It might help someone to create Notebookbar Mendeley Extension.
Insert Citation
Action: macro:///Mendeley.mendeleyMain.insertCitation
Image: %origin%/icons/insertCitation
Undo Edit
Action: macro:///Mendeley.mendeleyMain.undoEdit
Image: %origin%/icons/undoEdit
Merge Citations
Action: macro:///Mendeley.mendeleyMain.mergeCitations
Image: %origin%/icons/mergeCitations
Insert Bibliography
Action: macro:///Mendeley.mendeleyMain.insertBibliography
Image: %origin%/icons/insertBibliography
Refresh
Action: macro:///Mendeley.mendeleyMain.refresh
Image: %origin%/icons/refresh
Choose Citation Style
macro:///Mendeley.mendeleyMain.chooseCitationStyle
Image: no
Export MS Word Compatible
macro:///Mendeley.mendeleyMain.exportCompatibleMSWord
Image: no