interesting idea Webos! I havent seen a playdough "frame" before but can see the merit in it. I made a twin trunk frame out of wire a few months ago, maybe 9-12 months and have used C. elm for it. It has taken a massive amount of whips, something like 100 odd. and I still had some gaps. it grew pretty strongly. as we had a cooler period this year they got some form of a rest and now they are starting to bud out again.
I found it extremely annoying to try and attach one at a time, so I was putting three or four whips together (flat, not bunched) and found this easier. the biggest problem I have had is due to the size of the frame I made as I wanted it shohin (obviously) that the apex area got really thick, around an inch across on both apexes. I decided to leave it as I will cut back when they have fused as it will be easier to do later on.
you really need to give the roots a reasonable amount of space, ground growing is best and if you can cover a lot of the area you want to fuse it will come together quicker. in its next repot which is due in summer I will be only exposed in the top 3-4cm. if roots grow from higher up they can be trained down to fill in gaps.
I will get some pics for you of mine to show some progress. I beleive this technique is nearly like growing ROR, basically ya just have to wait for it to get some good fusing going on.
jamie
taking the top half of trees of since 2005! 
