On Polarhome, I used QNX, SunOS/Solaris, HP-UX, AIX and OSX. Having those running under qemu would be quite the challenge.

Until now, I have used qemu (or rather qemu-system-aarch64 in combination with binfmt-misc) on Linux to emulate e.g a Raspberry pi running on arm64. This works very well, but for e.g. Solaris or HP-UX there is the extra hurdle of getting hold of bootable media that will not freak out in the unfamiliar surroundings of a qemu virtual machine.

I have never tried, and it is possible that I overestimate the difficulty...