Anyway to display a popup menu from a button?

You might try looking at the NSResponder method -menuForEvent: — this would almost certainly require subclassing NSButton, but customized behavior and effects are among the reasons that subclassing is there in the first place.

 

NSPopover

 

Perform the following steps in Xcode (Xcode 4) or Interface Builder (Xcode 3) to attach a menu to a button:

1. Drag a pop-up button to the window.
2. Open the pop-up button’s attributes inspector.
3. Choose Pull Down from the Type pop-up menu
4. Choose a button type from the Style pop-up menu.
5. If you have an icon button, use the Image combo box to pick an image for the button.
6. If you have an icon button, deselect the Bordered checkbox.

 

Be sure to think through your rationale for doing this. There are many cases where this is a great UX feature, but it is potentially much less discoverable for the user and can lead to confusion. In many cases, it would probably be a good idea to make this functionality also accessible in the menu bar. It really depends on your specific use case, but I mention this because it may merit significant UX consideration. See Apple's HIG for further reference.