Keyboard hotkey for "Flush all caches"?
If you've enabled the search bar component on Admin menu you can search your way to it.




Enable the Search Bar component on the configuration page (/admin/config/administration/admin_menu).
Type "flu.." or enough characters to list only the "Flush all caches" menu item. 
Press the tab key to highlight and the enter key to trigger the "Flush all caches" menu item.