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...

Emulators can take you quite far, though you need to research some of them on the net to figure out working combinations of OS versions and emulator versions. Here are examples of things that I have managed to get to work at some point in time. Some for real software development and some for amusement.

KVM (x86 and x86_64): Linux, BSD, OSX, Hurd, Haiku, MSDOS, Minix, QNX, RTEMS, Xenix, Solaris, UnixWare, Windows 95 through 11.

QEMU (for non-x86): AIX 4, Linux (m68k, arm, sparc, powerpc, mips, riscv), OSX (ppc), Solaris 8 (sparc), SunOS 4.1.4 (sparc), Windows NT 4 (mips)

SIMH (for old DEC computers): NetBSD, VMS, Ultrix, RSX-11M, RT-11

Some of them can be quite finicky to get to work. Xenix was especially hard.

Solaris 11 is quite easy to get running in QEMU/KVM though. You can download the media from Oracle.

The only real hardware I routinely run has either Debian Linux, macOS, or Raspberry Pi.

Thanks for the summary! This looks more doable than I thought....