BTW, I've tried booting the recent venix boot disks. It's a bit slower than the real hardware, but not terrible.
However, there's some issues, but I'm not sure the root cause. It only detects 128k of RAM, but that may be due to a configuration issue on my part. It shows 128k in setup.
I create a disk with 'chdman createhd -chs 1024,6,16 -c none -o venix-rb.chd', and when venix goes to access it during install, it seems to hang. I haven't formatted it, so that may be part of the issue. The failure mode on real hardware with unformatted drives is an error about not being able to access a specific C H S.
You can find the Venix disks that I'm using at http://people.freebsd.org/~imp/venix86r-2.0.tar.xz. Root password is 'gnomes' if you get farther than I did. I've installed these on a real Rainbow 100B, so I know the images are good. The .flp files are raw images with no headers. vboot.flp and all xfer disks are bootable.
Btw, I'm running this on mac with 'mame64 rainbow -flop1 flp/vboot.img -hard1 venix-rb.chd' on a system that I'd previously gone into setup and saved.
These Venix disks recently surfaced. I've imaged them, installed them on my Rainbow, etc. I'll be blogging the experience and would like to have a section on running this under mame/mess.