I just spent the afternoon/evening, updating the install script to work on the latest Solaris release, 11.2. It's mostly working well now, though there are still plenty of limitations to using Solaris with Virtualmin (mostly just due to the age of the available packages, if you wanted to go through and install everything from the latest source, or package it yourself, you might be able to make a reasonably nice hosting experience on a Solaris box).
If you run Solaris anywhere with Virtualmin, feel free to give the new install script a try and file bugs when you find them (please be gentle, I'm not very Solaris savvy, as it's been over a decade since I maintained any Solaris boxes in the real world).
In my research today, I saw that Open Solaris is dead, and that IllumOS and several IllumOS distributions have sprung up in its place. I don't know enough about them to even begin to think about adding Virtualmin support to them, but if others out there are using any of those systems, I'd be curious to chat with you about them; particularly the package management situation.
You can find the new solaris-install.sh in the usual places (on the Software Licenses page in your account if you have Virtualmin Professional or on the Download page, if you use GPL).