As F4C4 stated, it doesn't work. But even if you were to use a powered external ODD, you would still be unable to share it using the Time Capsule. The USB port on the Time Capsule can only share a USB HDD or printer (or both, if a hub is used), and with the printer it will not work with functions other than printing (for example, scanning, faxing, etc.).
Serving USB devices over a network isn't as simple as just plugging it into a USB port on a router. The router has to be able to serve that device out to clients (print server, USB HDD server, etc.) in order for the clients to use it. USB devices are designed to be used by only one device at one time, so the router to which it is attached must be able to manage any protocols and queues that are required to make the device function as a networked device.
Even with Apple's routers, the TC and AEBS are able to serve both printers and USB HDDs, whereas the Express only has the ability to serve printers using its USB port.