My understanding is that that arena allows you to allocate memory segments, but you can't do much with it, you can't allocate var or object on it like in C++ for example, so its almost useless.

That Arena is not integrated into language. You can't do something like:

var myObj = new(my_arena) MyClass();

You certainly can, as they were designed as JNI replacement, with the goal to fully support the C ABI of the host platform.

You can either do the whole boilerplate manually with Panama set of APIs, or write a C header file and let jextract do the work of boilerplate generation.

> You certainly can

I am wondering if there is working code example, or this is just speculation?