I don't have any experience with Qemu, but assuming it can run x86 guests on ARM hosts at all, my expectation is that a old model smartphone would be incapable of running Gnome or Mate within a virtual machine with anything approaching decent performance, especially once you throw in the Accessibility stack.
Linux generally requires fewer resources compared to the proprietary OSes, but virtualization alone is a rather resource intensive application, and that's under the traditional assumption of guest and host having the same architecture, add in emulating x86 on ARM, my understanding that ARM processors have consistently been weaker than their x86 comtemporaries, and that Orca alone is a rather resource hungry application and is most likely going to want to be used alongside other heavy weights like Firefox, Chromium, and LibreOffice, and that heavyweight desktops like Gnome and Mate are generally considered moreaccessible than lighter weight options like LXDE, and Linux's resource usage advantage evaporates rather quickly. I'm not sure how the Raspberry Pi line of singleboard computers compares processor and memory-wise with the kind of old model phones under consideration, but even running things natively, getting Orca up and running on the Pi ranged from "impossible" to "proof of concept, too slow to actually use" until the Pi 4 came along and introduced models with more than 1GB of RAM, and even then, consensus seems to be 4GB Ram minimum, 8GB RAM recommended if you want to run Orca with Gnome or Mate and not having to be overly worried about memory usage.
