This is a simple extension originally built for use with Multilingual Zotero and the MLZ legal styles (normally used in conjunction with the Abbreviation Filter plugin). Never mind the buttons above: you can install the plugin by clicking on this link.
When the plugin is installed, typing Ctrl-s in the Prefix field of the word processor plugins will open a drop-down listing of the standard Bluebook signals.
Clicking on an item in the list will insert it into the field at the current cursor position.
And, um, that's it.