Hi Ed,
You would edit the Commandbar.htm file in MoI's UI folder to add a button.....

Here is a button next to max's custom UI..




The code to add a button is:

<moi:Spacer/>
<moi:CommandButton icon="icons/plugin.png" command="Subdiv6"</moi:CommandButton>

The icon is an image of your choice. The command is the name of the command....