Thunderbolt Display USB Clamshell Mode

I am trying to use a new Apple Display's (thunderbolt) USB ports while connected to my MacBook Pro (in clamshell mode). To be a bit more explicit below is the configuration that works and the configuration that I would like to know if it ought to work.

Works:

- Apply Display connected to MacBook Pro

- MBP is closed (in clamshell mode)

- external keyboard and mouse plugged into MBP

Everything works as expected. However, I carry my laptop around a lot and it would be nice if I could leave the external keyboard and mouse plugged into the display's usb ports.

Doesn't work:

- Apply Display connected to MacBook Pro

- MBP is closed (in clamshell mode)

- external keyboard and mouse plugged into Display

Is this just not possible? Is there something about being in clamshell mode that prevents the Display's USB ports from working properly?

Thanks!