To be more precise, I want to display a mirror texture that has not had timewarp applied - so actually I don't want to disable timewarp for the image displayed in the headset (though that would be fine as well for my purposes), but rather don't want to see it in the mirror texture.
EDIT: okay, on further reflection I should be able to just hack my app to display the submitted eye buffers to the mirror window rather than using the mirror texture, which should work for my purposes.