https://gcc.gnu.org/onlinedocs/gcc/Named-Address-Spaces.html

Unfortunately the obvious `__attribute__((mode(...)))` errors out if anything but the standard pointer-size mode (usually SI or DI) is passed.

Or you may be able to do it based on x32, since your far pointers are likely rare enough that you can do them manually. Especially in C++. I'm pretty sure you can just call "foreign" syscalls if you do it carefully.