ComboBox with CheckBoxes inside

[BCribbs]

The client has signed off on a UI prototype that the devs didn't get to see until today. Instead of a scrollable multi-select ComboBox, they were shown a single line ComboBox with each option having a CheckBox next to it.

Is there a way to do this in GXT? I figured a ComboBox could have a BaseTreeModel set on it but I can't find any examples of doing this online.

Thanks in advance,
Brian