Errr... why would you want a multi-select-option for command-buttons?
I understand multiselect, say in a listbox, but why for buttons?
Think about it:
You achieve your goal to select-multiple buttons in one go (either clicking consecutively with CTRL held down, or with a CTRL+A), then what?
Execute some code?
Execute the code within the Click-Events of each selected button? If Yes, in what order?
If you implement the multiselect in the usual way (aka what Endusers are used to), the moment you click on another button to execute something, all those selected buttons would lose its selection-status.
I'd think really hard if you really want to do something like that