There is no way to force portrait mode manually. What is the scenario where you would like to do this?
You can use the eye tracker alternately in landscape and portrait mode on a pivot monitor, if you first run a full EyeX monitor setup once for each mode.
Then, whenever you pivot your monitor, EyeX will detect that the aspect ratio has changed and automatically switch to using the setup corresponding to that aspect ratio. Of course this also implies that you have to physically move the eye tracker so that it is always attached at the bottom of the monitor screen.