I too like to use traverses as well as straight runs to look at where the knee is over the ski and their balancing abilities.
What I find very helpful is carrying a pocket full of cant wedges in varying degrees which can be easily slipped under the heel of the boot between the binding contact point and the boot sole. This simulates canting adjustments enough to get a rough idea of what is needed to achieve the disired results. The skier can feel immeadiate differences and the coach/instructor can see through the above exercises if the amount is appropriate and in the correct direction. These cant strips can be purchased from a variety of tool company sources and can be cut into many small wedges and the expense shared amongst the ski school brotherhood. You can use the old duct tape too but this method is slower and more of a pain.
I also carry 1.5 and 3 mm bontex rectangular shim pieces in my pockets to play with the fore/aft plane (delta angle). Again the skier can notice the differences and determine what feels best as well as the coach/instructor can see results too. These insole shims can also be purchased from your local boot fitter and are about a buck or two per shim and can also be cut into many usable smaller shims.
You may want to check with your area's risk management department before placing anything under the bindings. I personally feel comfortable placing no more than 3mm thickness under the bindings as well as cant strips under the heels without undo risk to release or retention of the system. I would also not take anyone in terrain that would present challenge to them. I keep them on groomed, open, intermeadiate or lower runs. Basically, use common sense.
I strongly believe that every instructor or coach should experiment with shimming overcanted and undercanted as well as positive and negative (to their current set up) delta angles, using shims, in a controlled setting on the same slope and conditions on the same day. This will create an awareness of some of the things their students may be feeling or experiencing related to equipment imbalances. It is a great eye opener and asset to improving your movement analysis skills.
All the above can be done on the hill without taking boots off. What follows has to be done inside and requires some special tools and skill so is probably not possible for the instructor coach to do with their skiers.
I also believe it is worthwhile to experiment with lateral shimming inside the boot under the foot. Please note that this is not canting (in the classical understanding of the term) but I believe it has merit in improving "balancing" and works inclusively with external canting. My opinion is these two areas compliment each other rather than replace the other. One optimizes "balance" the other optimizes "edge engagement". Together they provide strong effortless balance on a powerful biomechanically aligned edge.
But wait there's more....the skier's foot and ankle dynamics should be assessed as well as the whole body build to determine other fore/aft and lateral considerations when adjusting the boots. There is ankle range of motion, or more precisely, dorsiflexion range which affects choosing the proper net forward lean angle (forward lean - ramp angle = net fwd lean). That has to then be assessed in the ski binding system to include the affects of delta angle on the whole system. We can then assess static balance using various formulas and theories which all come to pretty much the same result of having the knee plumb pretty much over the toes and the com over the center of the foot.
Oh, and before all this it is not a bad idea to make a custom footbed to cradle the foot in a "soft neutral" which will offer comfortable support from being crushed by the vise like buckling yet allow the foot to pronate enough to facilitate the foots incredible ability to balance our centers of mass while in motion on skis.
Or you can simply ask your technical director, Mr. Barnes. I here he knows his stuff!