Try setting computer sleep to 'never' in energy saver, and connect an external keyboard or mouse. Close the lid, and click mouse or keyboard key to wake unit if it goes to sleep. This should trick the MacBook Pro into "clamshell" mode. You might want to put the unit on a small riser or laptop cool pad, as it is going to get warm...
Check out this article from Apple's Knowledge Base:
http://support.apple.com/kb/HT3131
PS: You shouldn't need an external monitor connected for this to work.