Agreed. It doen’t seem to be available in the control assignments. I would like to change it to my other controller (Reverb G2) but can’t do that, it seems to be a fixed assignment.
However… There was a thread around before SU7, where someone had found the VR controller assignments in one of the input XML files. That may be a way of assigning this control manually.
I haven’t got time to dig further right now, but I might be able to find the thread which might help. Will edit my post with the thread if I find it.
EDIT: here it is: VR Controller support is missing. Please add in addition to 3D mouse - #239 by CptLucky8