Today, I decided to try the QEMU CPU emulator. I installed it and, after fighting several problems caused by incorrect flags passed to the compiler, I got it working.
So I tried to install OS/2 Warp 4 as the client OS. The emulator is very fast, compared to others such as Bochs (yeah, well, they don't work the same way). I had some problems during the installation (strange crashes in the installer) but, after disabling the network installation, it seems to work properly now.
Hmm... I haven't been able to run this OS in any other emulator :-) It's really worth trying.