Devices already connected to a Chromebook when locked will continue to work, ensuring, for example, that data transfers do not stop. Once the functionality is provided, you should be able to choose to use the service and whitelists of USB devices like a keyboard or mouse.
Chromebooks are considered more secure than Windows and other devices, especially for distraught users (I'm talking to you, Aunt Gertrude with six spyware toolbars). However, any computer can be vulnerable to Rubber Ducky devices, essentially miniature computers that look like keyboards and inject preprogrammed keystrokes at a maximum speed of 1000 words / minute. All this can be stopped simply by disabling the USB port. The USBGuard feature, which will soon be available for Chromebooks, is therefore welcome.