Personally, I have been using a Display port to play Mass Effect 3 on my PC.
I have two monitors though, one connected via DVI and the other using the Display Port.
When I play Mass Effect 3, it will launch, full screen, on my main display, the one set-up as screen 1 from the Screen Resolution menu (Right click on the desktop).
I used to have my DVI set as the Main Display, and when I wanted to play Mass Effect 3, I would have to switch the screens first. (Now I re-arranged my desk to where the DisplayPort monitor makes sense as the main display)
Even though you are mirroring the video feed, it might only use one at a time, for performance reasons.
Question, when the screen goes blank, does your HDMI device go into self test mode? The mode where it informs you there is "No Input", but it is on.
That would be the difference between the video card disengaging the HDMI connection, or sending just a blank screen.
On my laptop, I can press the Fn and F5 key to switch between displays (use just VGA, use both main screen and VGA, just main, etc...)
I recommend performing a similar operation on your laptop such that only one display is active, your HDMI connection, then start ME3. Be ready to hit that combo again to bring it to your main display in case it doesn't work!
