AusBonsai

We used RCBs for growing advanced landscape trees at a commercial nursery. They do not cause more roots to grow from the trunk base any more than any other root control method. The fine roots can penetrate through the bags so they have some access to the round outside the bag but as the roots thicken they are constricted and are cut off. New roots then grow but usually just behind where the root has been cut off - just as we see when roots are pruned with scissors.
When you open up a RCB you find hundreds of roots with swollen lumps just inside the bag. The lumps are callus where the roots have healed after they stopped growing because the fabric constricted growth. It is usual to find lots of root ramification near the ends of a few main roots rather than many roots growing from the base of the tree but that depends far more on the species, the time in the bag and growing conditions than the fact they are in a bag.
Best of luck with your trials. I don't bother with RCBs for field growing bonsai. I found that good root pruning technique produces much better trees more often in my grow beds.

Some photos of sacrifice branches on pot grown shimapku Note that the sacrifice branch is at the rear so it won't interfere with the tree or show up when it is removed. It is also closer to the top of the tree and should thicken the whole of the trunk below it. You may also see that the apex of this tree, above the sacrifice branch, is starting to get weak - a sign that the sacrifice branch is taking too much and needs to be suppressed or removed.
Another shimpaku I could not resist moving to a bonsai pot. This time the sacrifice branch is really low so it will only add thickness to the base of this trunk. There were 2 parts to this sacrifice but the rear section was so big the tree would not stay upright so I had to remove most of it. That branch will probably become jin when it has achieved results or when I get sick of waiting.
This one is being developed as a shohin sized bonsai. Currently has 3 large sacrifice branches. I tried drawing my plan over the photo but the file is too big when I add some paint to it. The branch on the left will become a short jin. Lower left branch will be shortened a lot or jinned completely. There's a small shoot coming from front left of the bigger right trunk. I'm planning to use it as the apex of the final tree. The remainder of that large trunk has been wired and curved so I can convert it to a long, curved jin to see what that looks like.
Another one aimed at a smaller, shohin sized tree using the central twisted trunk. The large sacrifice branches will probably become short jins.
Cascade style shimpaku. I thought the upper trunk needed to thicken a little so that upright branch midway down the trunk has been allowed to grow. i'll need to convert it to jin so that has meant leaving it to grow bigger. When you remove the bark there's not much wood inside and skinny matchstick jins don't look good so I'll let it get to a reasonable thickness before making the cut.

Another very helpful post Neil, thanks. You're a star. SA is lucky to have you visiting in the near future.

Thanks for the helpful advice shibui, I didn't really know much about sacrifice branches until this post but you explained and outlined your plans brilliantly.

Any updates on these?

Many of these look similar but a little thicker maybe - junipers are slow to develop.
I keep wanting to remove the sacrifice branch on the cascade but keep convincing myself it is still doing good work and should be left longer.

The one in the round pot had an accident and features in this thread:
viewtopic.php?f=131&t=28117&p=274854&hi ... er#p274854

That’s one gnarly looking Jin I must say will be interesting to see how it develops

Try this method for growing shimpaku 'on' the ground:
https://www.youtube.com/watch?v=sXu6uScJewo
I'm planning to try it.

Any updates on these RCBs?
When we used them for growing advanced trees at a commercial nursery they did some of the things claimed but I don't think I would use them to develop bonsai. Maybe I'm wrong?

While we wait here's another shimpaku sacrifice branch that seems to have done a good job of thickening the trunk. I don't want to give the impression that a sacrifice branch will suddenly thicken the trunk. This one has been growing for around 10 years, maybe even longer to get to this stage.
The long sacrifice branch keeps on overbalancing the pot so I decided it must have done its job.
Note that I've only shortened it for a start. Removing really strong branches in one go can cause roots and part of the trunk to die as they are suddenly deprived of sap flow. I though it may be prudent to go a bit slower with this one seeing as the sacrifice branch had more foliage than the rest of the tree.