Installed base, for our purposes, is two things:
1) pages in the databases.
2) rules in people's heads.
The first is easy to fix, you just grind.
Grind what? Is there an exhaustive list of all MediaWiki installs
somewhere? I very much doubt it. We can test against WMF and Wikia
databases, and maybe a handful of others, but there's no way we can
check every install.