Will a MacBook Pro (15-inch, Mid 2010) power up in clamshell mode?
An easy way to spoof a monitor connector is to use a VGA adapter, and connect a VGA Dummy-plug (sometimes called a "VGA Terminator").

There are a number of tutorials on how to make your own - here and here are other (though these involve soldering).

You can also buy them in DVI-A and VGA flavors.

If you want something that spoofs a DVI-D interface into thinking there is an attached monitor, it is a lot more work.