There is sim/cockpit/autopilot/vertical_velocity which is a writable float dataref for "Vertical speed to hold".
I assume this is the default X-Plane dataref for vertical speed, but it's ignored by the IXEG. I therefore also assume that the IXEG is using it's own internal "dataref" which is not exposed to the outside world.
I think this is one for Tom to have a look at.