prgmr.com has the openbsd installer officially available, as well as the netbsd installer and freebsd (both as an image and the installer.)
For those of us in Europe there is also https://openbsd.amsterdam/ (I mention them because one obvious reason to prefer DO is that they have locations outside California).
I wonder if this instructions would work for NetBSD as well, just replacing the installer…
Sure it’s easy. I wrote about FreeBSD on Linode a couple of years ago and published a script that installs FreeBSD/OpenBSD/NetBSD based on prebuilt images.
The most obscure OS I got working was SmartOS. The most obscure OS I heard someone got working was Windows 1.3.
I’ve got some OmniOS droplets running from a hollowed out FreeBSD droplet, too. Once you get the image/snapshot made with appropriate scripts to read the metadata it’s pretty easy to stamp out more.