Let me share with you my failure of my week-end:
I tried to improve the Zento-Kayou by drilling a hole at the end hoping that I could refill the lead without disassembling the pen each time. The body is plastic so it was easy…
… unfortunately it doesn’t work. The lead falls in the cartridge but doesn’t engage if you don’t push it all the way in[1]. Opening the pen is still required to refill lead. Oh well, now I have a Zento Signature with a hole 
(it was already quite worn when I got it so I’m not terribly sorry about this).
However, there is a silver lining. Along the way I found that the front grip had a little wobble so I investigated and managed to disassemble it:
The nose cone attaches to the metal core and holds the grip and the silver ring in place.
Tightening the cone while holding the rear thread solved my wobble problem.
Regarding the materials used:
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the core is chromed metal (non-magnetic, probably brass, maybe one day I’ll scratch the coating to be sure)
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the ring is probably steel (it’s the part that attracts the magnet in the cap)
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the grip is some hard plastic/composite, probably the same as the rear body
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the nose cone is painted brass (the golden colour is visible in the unpainted inside)
So at least I’ve learned something.
I guess that using standard 60 mm lead (once the provided 90 mm ones run out) will probably require a second lead to push the first in ↩︎