This is what i foound. WorkSpaceTool uses .dat files as icons and you can find all icon .dat in the icon folder. In order to create your .dat you need to create it using the following script:
https://svn.blender.org/svnroot/bf-blender/trunk/lib/resources/
once you’ve create .dat put it the icon folder and bl_icon = ‘dat_file_name’. Hope it helps.
Edit: for reference Creating costum icons