Okay, I guess from my perspective I feel a gymnast performing BWO on beam should not be shifting the weight forward as this is a very big mistake in the BWO. I don't typically see this problem on beam. When I teach BWO on floor I don't think the way I spot allows for it, however when they go by themselves I often might see it especially if the kid is afraid. Then they don't want to go back in the correct position to push the shoulders over the hands. This is the main issue with shifting the weight forward, the shoulders won't go over the hands (head often out) and in the case there isn't enough power for the skill to go over straight.
I teach first a bridge (lift arms and legs) then a bridge kickover on beam (this is after they can do candlestick and a back roll). Once they can do the bridge kickover I'm pretty confident with their momentum and direction. If they can't push shoulders over hands in the beam position then we keep working on it because that isn't a good sign. However the next drill I still feel is important for stability and that is to do the first part of the skill into a bridge (no kickover). Basically the front leg will lead back as with a back walkover but when the hands hit the skill "freezes" so they end in a bridge with one foot straight up. This may be best done on a softer surface but can even be done on the beam by more advanced gymnasts. Look for the leg that stays down not to "buckle" with the knee going over the ankle as the kid starts to arch back. This can be done with the leg lifted but does not have to. When they can do this without that issue and can do the bridge kickover, there should be no physical problem with performing the skill with no weight shift and they can point their front foot out in front which lends some balance and stability at the early level of this skill (I agreed earlier that lifting the leg would be ideal, however I would not require this of my level 6s, especially in competition routines).
But basically if they try to make any adjustment to slow the skill down it probably won't go over straight unless they have really above "tight" flexibility. The rest of us need to get the shoulders over the hands as quickly as possible. Buckling the support leg is a direction error with the momentum in the skill, much like doing that out of a RO would cause the head to go back and shoulders probably to collapse on a BHS. So in essence I agree it is true that standing on one leg is more likely to make them push through their leg and show stability which will avoid the problem of the incorrect shoulder angle.
However I think there is some merit in keeping the dominant leg pointed in front for the beginning performance of this skill on beam. I would also recommend breaking down the skill as above, and also make sure they can do other backwards skills on beam (back roll, backwards bridge walk, bridge and lift arms and legs) before doing the BWOs and BHSs. This way they can isolate each part of the element and learn which muscles stop and start the positions (i.e. doing the back bend to "freeze" on one legs, and pushing the shoulders over in bridge to do the kickover). Also train a lot of split handstands to lunge, lever, lever to jump (using correct punching action for beam series), and lever to walk back on toe.
This will give them more confidence in correcting and controling the positions the skill can end in, and also lets you train that end part many times with less stress on the back, which I think is the most important thing in our beam training. If they can't do good, even split handstand (parallel to beam, not necessarily perfect 180, but even) with tight legs (you'll see a lot of girls have the semi bent leg, the easiest way to fix or improve this this is to point harder through the toe) then they are going to struggle with the beam acro.