I don't think it's a good idea to clutter the shelf with a lot of buttons. We haven't received other requests for show/hide/select buttons and most likely different users need different customizations; we would end up with a million buttons.
However, it's easy to do what you wish with MEL. For example, this command selects all Maxwell references:
Code:
select -r `ls -type maxwellReferencedMXS`
If you type this in the script editor and select the text, you can drag it with the middle mouse button onto the shelf and Maya will create a new button which runs that command. You can also use the shelf editor to add, edit and remove buttons (you will find it either in the Window -> Settings/Preferences menu, or by clicking on the down arrow on the left of the shelf).