The trouble with symbolic links

A unionfs of thousands of filesystems is going to be hilariously slow. You're replacing an O(smallnum) lookup when a symlink is resolved with an O(n) where n == number of installed packages for *every file lookup*, as it works through all the layers.

Or did you mean to suggest some other, cleverer trick that I hadn't thought of? (Quite likely, because the naive approach is obviously a disaster.)