I use a Japanese keyboard for that very reason (even though I don't speak japanese). I don't think that, technically, it's an "hyper key". These are the henkan and muhenkan keys, to switch input methods (kanas / romanji / etc.).

But that's precisely how I use these keys: I remap the one at the left of the (always very narrow on japanese keyboard) spacebar to "Hyper".

Note that the USB HID doesn't have the concept of the Hyper key (so I remap it from X11 / xkb).

Now... Some may wonder: why buy a japanese keyboard and not some fancy ergonomic / DIY keyboard? Several reasons. Japanese is kinda a big country and japanese keyboards are here to stay, so my trick shall keep working until long after I'm dead. Japanese laptops: japanese keyboard on the laptop, I now effectively can have a laptop with a "Hyper" key.

And then the main one: the DIY keyboard community can apparently only do variations of Cherry switches or similar switches. My switch of choice is Topre. I didn't find any DIY keyboard with Topre switches.

So I use a japanese keyboard, with Topre switches. And I remap it and there's nothing "japanese" about it anymore.