In the case of Watchman, I have to assume that internal use is the most supported use case, and uniformity of deployment is desirable across the fleet there, and so, configurability wasn't as big of a concern?
So, based on the responses from the maintainers, for the .linuxbrew directory to be used, you have to satisfy 2 conditions:
1. you have to be installing one of the ~10% of formula that isn't trivially relocatable.
2. you have to be using a precompiled binary (which it seems like homebrew is smart enough to not do if condition #1 fails and you're not using sudo)