How about rendering the player avatar as a 'cockpit'? Have it so you can see you legs and arms - then you know which way forwards is.
It's more than about 'knowing' your orientation and position, you really need a continually visible stable frame of reference to avoid the nausea. A mechwarrior2 cockpit works fine, where as an avatar's arms and torso suspended below your view really isn't enough.
On top of all that there's actually a fair bit of wonky stuff that the brain does simply from the visual/tactile height mismatch when attempting to view a virtual standing first person view from a seated playing position: http://www.gdcvault.com/play/1020714 @27min or so. Between the problems of nausea, locomotion and scale it kind of makes sense why Valve seems to have ditched the idea of porting existing seated fps content with just some control tweaks and have instead committed to 1:1 tracking of every form of motion with custom tailored content. As the vr hmds get more advanced each year and our brains try to treat the images in an increasingly natural way we seem to discover new, non-trivial hurdles. Perhaps VR just won't quite work as simply as we might have assumed only a few short years ago.