You can do this by creating your own plugin buttons. You can name them how you want, specify size and even have an image if you wish. Basically a button you create presses an interface button, so you leave the interface buttons in their palettes and use your buttons in the custon UI.
Here's how it's done:
The ISubPalette line creates a new menu in the ZPlugin palette. You can leave this as it is.
[IButton,"ZPlugin:My Buttons:i","Initialize ZBrush",
,,15 /*width*/,, /*image*/, /*height*/]
Each button starts with its name, which for plugins must include the palette and menu. In this example the full name of the button is "ZPlugin:My Buttons:i" but only the "i" will show on the button. The "i" is all you need to change for any new buttons you want to create.
The VarSet,buttonPath line sets a variable to the button path for the interface item you want to press. You can find the button path for any interface item by holding the cursor over it and pressing Ctrl. The button path is given at the bottom of the pop-up.
Once you've given your new button a unique name and set the buttonPath all you need to do is set the size. You can leave the values blank but I've specified 15 for the width. Any value >1 can be used and is the width or height in pixels. Values of <=1 are treated as proportional to palette dimensions, so a value of 1 instead of 15 would give a button the full width of the palette.
I've attached the code in a text file. Save this to your ZStartup/ZPlugs folder and then load it through the ZScript>Load button. Once you've done that the buttons will be available every time you start ZBrush and you can make them part of you UI.