The mouse on this system (per comments above), a Microsoft USB "Basic Optical Mouse", was replaced with a Logitech Wireless M505 (which uses the Logitech Unifying USB receiver) and now exhibits a new behaviour: the system still hangs on boot-up but as soon as the mouse is moved the system continues booting (no Escape key presses are needed).
The mouse on this system (per comments above), a Microsoft USB "Basic Optical Mouse", was replaced with a Logitech Wireless M505 (which uses the Logitech Unifying USB receiver) and now exhibits a new behaviour: the system still hangs on boot-up but as soon as the mouse is moved the system continues booting (no Escape key presses are needed).