So, why is sparc64 unusable, and what are the barriers to creating a fully usable sparc64 emulator which can run Solaris 8/9 (sparc64)? ie, how far away is it and what resources are lacking?
There are bugs in Qemu's Sparc64 CPU implementation that need to be found and fixed. Those prevent the OpenBIOS (or even simple C programs for the easier sparc64-linux-user case) from running. This part is just debugging, most things are in place.
When the CPU core is fine, OpenBIOS can probe for PCI devices, but the PCI bridge is not complete. Any real kernel would want to enable interrupts, but there is no interrupt controller yet. And so on. But implementing these is not very difficult.
Also OpenBIOS needs some work before any kernel will run, for example in the device tree and client interface. Should be easy.
For your specific system call problem, maybe you could implement a Sparc64 user emulator for Solaris, like the Linux and Darwin emulators. That would help by exercising the CPU core and finding the bugs.