I suggest you may plant these directly to soil without pots and wait for a few months so that enough trunk thickness is obtained. The cut the trunk at desired height. Do one more root pruning and wait for few months again. Then you will have a bonsai with good looks. Only after these you need to start root over rock style planting.
You may suggest that. However I am not going to do that, as that results in a few problems, making me wonder why you give this advice at all:
This type of ficus may resist the odd night-time one degree of frost sprinkle, resulting in foliage loss, but will recover if temperatures rise again. Extended periods of freezing, let alone a few weeks of -15C are not an option.
As discussed in
another thread
I am trying a few different methods of creating a nice trunk. In this case these I will let grow a trunk from the rock itself, as this is a plant known to create unbelievable roots & trunk with which they break rocks. So I think it will be OK browing in confined space while at the same time let the trunk grow enoug te become convincing in a few years. (see
www.plantzafrica.com/plantefg/ficusabuti.htm)
Remember: Each species is different. Personally, I do like to get quick results, but I would also like to try and go for some potential stunning trees. I am still convinced the 'let het grow and chop' method is not ideal for that.
Finally, when giving people advice, try to remember: This is an international site. People are all over the world. Without knowing where they leive, what their climatic conditions are and which the requirements of their tree-species are, be carefull what you advice people to do with their plants.