I placed some bamboo in and wondered how to tie it together. Screws are ok, but may work loose. I did what you are thinking about doing. I drilled some holes and used barb wire to tie them together. Will take a long time to corrode that wire.
My condo's---I got the idea from the videos that you posted. The willow is great and I use the sand bags with rocks in them. I got a 100 bags for about 35 bucks. The jug in the top also helps hold the tree upright.