You might read this question (and its answers), which I posed in May 2011 and have not yet received a satisfactory answer for. The short version is: Optimus allows mixed-mode rendering by allowing the powerful Nvidia GPU to write (e.g. output video) directly into the frame buffer of the Intel IGP.
Based on my experience with optimus, it is possible that the Intel graphics adapter is hard-wired to the HDMI output port on your laptop with a second frame buffer that is not accessible to the Nvidia GPU. If this is the case, it will not be possible under any circumstances to use the Nvidia GPU for that particular output.
To confirm this, I would use the Nvidia control panel to force GPU rendering of the application you want to run, and then confirm that the GPU is used for that application on the laptop's built-in screen. Then try the same thing on the HDMI port and check that the GPU is being used. If running the application on the monitor connected to the HDMI output results in the GPU not being used, you are most likely out of luck.
You can use the Nvidia Optimus State Viewer to determine with certainty whether the Nvidia GPU is being used.