I dis-assembled my Hitachi to see if I could find you part, and how it relates to the other pieces.
It is rather interesting on the inside. I don't think I have had this one quite this far disassembled before (on a side not, I think I found the source of the funny noise/vibration I was getting, there is a spring only attached at one place )
This is what the tensioning linkage assembly should look like (except for that pesky spring)
This shows the top pieces of the assembly. The handle thingy has threads on the inside of the tube which thread to the long rod. The other tube is just a spacer bar, no threads.
I believe this is the mysterious triangle piece that you are missing.
In my saw it has no threads and basically just acts as a spacer/alignment piece for the tensioning rod.
The way it looks in my saw, if you could find some sort of material that could withstand the compression of the tensioning, and would fit in the notch of the arm, it would work.