Add item to Finder toolbar with terminal?
This is as far as I’ve got, and it’s not working for me on El Capitan (though, that could be something to do with the beta…).

It seems that defaults isn’t comprehensive enough (anymore?) for you to achieve what you want, however you can use PlistBuddy, which on 10.11 is currently found under /usr/libexec/PlistBuddy, to add/remove/edit plist files.  

Here are the two commands you’ll can use to add the _CFURLString and _CFURLStringType:

/usr/libexec/PlistBuddy -c 'Add "NSToolbar Configuration Browser":"TB Item Plists":8:_CFURLString string "file:///Applications/Chess.app"' ~/Library/Preferences/com.apple.finder.plist

/usr/libexec/PlistBuddy -c 'Add "NSToolbar Configuration Browser":"TB Item Plists":8:_CFURLStringType integer 15' ~/Library/Preferences/com.apple.finder.plist


The “8” found in both of the commands above is the array index of the item.  

No _CFURLAliasData is generated (as suggested in several places).  I’ve tried creating an empty _CFURLAliasData, but Finder didn’t populate it.

Another issue is that if I add/remove another item using cmd-drag all the data I’ve added with PlistBuddy gets erased…

Hopefully this information will get someone else partway there.