I use rebar pounded into the ground at a slight angle. I was thinking that this year I might weld up a couple of rebar "Y"s so that I only need to pound in one rebar between two arches.
In hindsight, I think next year I will drill/tap some screw holes so I can tighten screws in against the rebar. This will give me better "angles" to the ground. (live and learn)
I doubt the material is strong enough to have tap screws that will be helpful. There's an awful lot of leverage on an arch where it connects to the ground, and I think any screw holes would only weaken the material at its most critical points.
Why not use hose clamps instead of zip ties? Much stronger, adjustable, and you could continue with attaching the arch to the external rebar.
Your idea of a hinged section makes a lot of sense, and fastening it to the top of the pole can be done pretty easily. Here's the top of my midi-tree (about 12' high). I wanted to make it removable so I could take the light collar off the pole and store the lights separately from the pole, but the concept could be similar to what you could do.
I drilled 3 holes all the way through the main pole and put three large hook eyes through it. The top collar (4" pvc) fits right over it and sits inside the hooks. The multiple holes in the collar are for zip ties that are used to hang the light strings from the collar.
Obviously, these pictures are horizontal as I was making the pieces, but it's not much of a stretch to see how the collar just sits down into the hook eyes. Your hinged collar could work much the same way. You could hold it together with a hose/duct clamp and still have plenty of room for lights.
Heated with an oxy/acetylene welding torch. Get a spot red, and it bends very easily.
Ahh - - perfect oppportunity to show off and say "With my bare hands - what else!?"