I don't know if the pipe would work as you effectively as once the plants fused and hit the edges you would end up with a perfectly straight round trunk with no taper. Something I've noticed on this one with the plaiting or twisting is bulging areas and from some angles it looks quite flat. These can be used in the design but worth being aware of.
I started another last October, with about 10 cuttings of a QLD Small leaf. I squeezed them together and wrapped them with grafting tape. They seem to have fused OK, some areas still need a bit longer but i'd say they'll be fine come this October. From there I intend to root some cuttings, place them around the fused trunk and wrap with grafting tape again. Rather than having all of these go straight up in the air, I'll bend them out where I want my branches to be. Confused? Me too
I'll take a couple of pictures when I get the Benjamina one for you and Paul.
There is a really good article about a Gede Merta tree which I think was in Bonsai Focus. It is what got me thinking more about this. It's similar to the link below but Gede began with a living trunk and I think just added branches.
https://shanemartinbonsai.wordpress.com ... project-2/
Cheers,
Nigel