For example, if I wanted to add a button (say, next to Edit With…) that displayed the current font's notes in a new window, would that be possible?

that is easy

from mojo.UI import CurrentFontWindow
from AppKit import NSImageNameAdvanced

class AddToolBarButton(object):
    
    def __init__(self):
        # maybe subscribe to the "fontDidOpen" event
        
        # get the current window
        window = CurrentFontWindow()
        if window is None:
            return
        # get the current toolbar items
        toolbarItems = window.getToolbarItems()
        
        # make a new one, see the vanilla docs for more info
        newItem = dict(itemIdentifier="myNewButton",
                label="Button",
                imageNamed=NSImageNameAdvanced,
                callback=self.myCallback)
        # add the new item to the existing toolbars
        toolbarItems.append(newItem)
        # get the vanilla window object
        vanillaWindow = window.window()
        # set the new toolbaritems in the window
        window.toolbar = vanillaWindow.addToolbar(toolbarIdentifier="myCustomToolbar", toolbarItems=toolbarItems, addStandardItems=False)
        # done
    
    def myCallback(self, sender):
        print "hit, do something!"
        
AddToolBarButton()

good luck!

Is there a list of the observable events that RoboFont supports? I assume that I can just read through the source of defcon to find its events...

see:
http://doc.robofont.com/api/custom-observers/
http://doc.robofont.com/api/custom-tools/

My ObjC abilities are poor, and I'm not sure if this is a job for NSPDFImageRef or…?

from lib.UI.toolbarGlyphTools import ToolbarGlyphTools

newItem = dict(itemIdentifier="myNewButton",
        label="Button",
        imageObject=NSImage.alloc().initByReferencingFile_(imagePath),
        callback=self.myCallback,
        view = ToolbarGlyphTools.alloc().init()
    )

not sure how to do code tags either…

The ToolbarGlyphTools is an UI object that looks like a button that you can add in a toolbar dict item.

usage:

from AppKit import NSImage

from lib.UI.toolbarGlyphTools import ToolbarGlyphTools

## create your image, best is that the image is only black and white
## so the image can be inverted when clicked
myToolBarImage = NSImage.alloc().initByReferencingFile_(imagePath)

## a ToolbarGlyphTools is similar like the vanilla.SegmentedButton but has an extra toolbar item attributes
## if you want to add more items to the segmented button the item attributes just contains more dicts 
## the size of the segmented button is atomically multiplied by the number of items.
toolbarView = ToolbarGlyphTools((30, 25), 
        [dict(image=myToolBarImage, toolTip="My ToolTip"], 
        trackingMode="one")

newItem = dict(itemIdentifier="myNewButton",
        label="Button",
        callback=self.myCallback,
        view=ToolbarGlyphTools.alloc().init())

good luck

use <pre>your code</pre> to get the syntax highlighting in a post

I've written a proper vanilla patch which is already in the vanilla repo. This will be in the next update of RoboFont.

It adds addToolBarItem and removeToolbarItem to a vanilla window object, able to manage toolbar items properly.

see https://github.com/typesupply/vanilla/blob/master/Lib/vanilla/vanillaWindows.py#L631