A sleeve wont do anything, the bearings are put into the hub from the outside so a sleeve on the inside can just push it out until the axle contacts the bearing that is being pushed into the hub. It effectively does nothing unless the bearings are captured, then the two bearings would share the load (only when pushing out) but since there is play the cross pin (I am assuming that you are talking about the one behind the hex) will still get all the load of the wheel nut being tightened.

I dont see anything wrong with the design, there are no reports of premature bearing failure and most bearings that are getting damaged are probably getting damaged more from the inhospitable environment they are being run in rather than side loading.

Also, I just checked a bunch of manuals for similar vehicles and 0 of them have a crush sleeve.