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.
var myObj = new(my_arena) MyClass();
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.
I am wondering if there is working code example, or this is just speculation?