Are there any plans to support this setting in the future?
I've only seen Einstein@Home with this setting so I'm not sure if it's something they invented themselves or whether it's part of the general BOINC server code.

I'd like to be able to crunch CPU-only tasks (n-body) when they're available and then switching to tasks, with CPU and GPU versions (de_separation), only when/if the CPU-only tasks are unavailable. I'm only using my CPU on this project as I'm suffering overheating issues when I use my GPU here. If I sort out the overheating issues then I'd like to be able to crunch the de_separation tasks on the GPU only and n-body tasks on CPU only.

Your concern would also be solved if all CPUs received N-Body only. In addition I would think that this would benefit the science by allowing the separation WUs to be more quickly adjusted to new parameters instead of waiting many hours for CPU WUs to be completed.