Hi, that is really great, and I appreciate not only the work you did but also you being just in time for me merging my separate wikis to a farm. I like especially that it does not force one to a special configuration; instead you can use wikis within the same or on different databases, and the same applies to the virtual addresses as you described.
Before I start using it, I have one question left though regarding the shared directory structure. I know how to set up separate /images directories per wiki if needed, and it is nice that I do not even have to think about symlinking /extensions, but the question is about directories like /cache. In former versions of wiki farms (e.g. with symlinks) it was documented that it shall not be shared between wikis. Did that change? I did not see any code related to that topic in your solution, but may have missed it. Do you have any (bad/good) experience with sharing the cache folder?