There's no requirement that Homebrew be in /usr/local. Some individual (poorly-written) packages may assume that, but I've been running Homebrew in $HOME/.brew for multiple years on multiple machines with no issues at all.
That's actually fucking brilliant, and aligns much better with what homebrew is. I'm setting up my new macbook from work tomorrow and this is absolutely the way I'm going to do it, and how I'll suggest any of my colleagues do as well.
