My Haflinger has the same stability issues in areas of steep slopes and deep holes (wombat holes) and for offroad work I am also considering wheel spacers to assist with stability - yes the pinz and and the haflinger have great ground clearance because of their portals but that comes at the expense of stability in some circumstances - not helped by the swing axle design - so with advantages come disadvantages.
I also have a Landrover FC 101 - basic suspension and no portals but I do have diff locks. I have been 4wding in it with a couple of Pinz on a few occasions. The vehicles overall are very similar in offroad capabilities - the Pinz has better clearance under the diffs (though not as bad an issue as you think) but the 101 is far more stable because of its lower centre of gravity and solid axles so sideways it is better than the Pinz.
However where there are holes that you can drop a front wheel into - both have the same issues - lifting back wheels high in the air and if going down hill easy to tip - I have nearly gone over a couple of times.
So in my mind with all three vehicles the solution is to widen the track as far as you can within reason - load issues may come in. On my 101 I have increased rim width from 6" to 8" an increased the offset of the wheel so that the track has increased by about 1 1/2".
With my Haflinger I have 14" rims that are 6" wide so increasing the track but this is offset by the larger diameter tyres so remains unstable offroad. In this pic you could imagine what would happen if my front wheel went into a hole on a slope. For offroad I am also looking at spacers and ATV rims and tyres.
So for the Pinz, I think the preferred option would be wider rims with different offset to increase the track but spacers would also work as long as they do not over load the hubs.
However driving these cab over designs with this tip tendency in mind goes a long way to reducing the chances.
Garry